Make `xmd plan` produce approved XMD source only

[taras]

Story

As a command-line user, I want xmd plan to produce one reviewed XMD program
artifact and never execute it, so I can choose explicitly when and where that
program runs.

Common paths

Write the exact approved source to stdout:

xmd plan "Prepare the release program."

Compose planning and execution explicitly through standard input:

xmd plan "Prepare the release program." | xmd run -

Or preserve the artifact first and run the saved file later:

xmd plan "Prepare the release program." --output release.md && xmd run release.md

--output remains an exclusive file sink: it creates the requested path only
after approval and never replaces an existing file. Without it, stdout contains
only the exact approved program bytes.

One command responsibility

xmd plan maps one instruction string to reviewed and approved XMD source. It
uses the same packaged <Plan> workflow as an ordinary document and does not
start a second root after authorship.

The packaged plan-command document remains a thin adapter: it supplies the
command surface to <Plan>, captures the resulting text, and returns those exact
bytes. The host awaits authorship and teardown, performs the existing final
caller-owned checks, and sends the artifact to exactly one destination. No
approved program effect runs on any success or failure path.

Command grammar

Remove --run completely; it has no alias. --run, --run=true,
--run=false, repeated occurrences, and any placement of that spelling receive
one actionable migration refusal before a catalog, Agent, session, review,
output file, journal, or document execution exists:

xmd plan --run was removed because xmd plan only produces approved source.
Run the program explicitly:
 xmd plan "..." | xmd run -
 xmd plan "..." --output release.md && xmd run release.md

Retain only arguments that configure planning or artifact disposition:

• the one instruction argument;
• --include for the syntax catalog;
• --agent-provider and --default-agent for authorship;
• --session for the planning conversation;
• --timeout for the whole planning invocation;
• --output for the exclusive artifact sink; and
• ordinary --help and --version behavior.

Remove every option that configured the approved program's former execution:

• generated root-property arguments: --props, --props-*, and
  --no-props-*;
• --raw;
• --verbose and -V;
• --journal and -j;
• --timeout-exec and --timeout-fetch;
• --approve-all, --approve-reads, and --deny-all; and
• --secret-detection and --no-secret-detection.

Each removed spelling refuses as an xmd plan option before authorship. The
corresponding xmd run options remain unchanged. #676 later introduces the
long-form --verbose and --journal <path> options together with their complete
authorship presentation and diagnostic behavior; this story does not accept
inert placeholders for them.

Update command help to say:

Turn a request into an XMD Plan, review it, and write the approved source.

Help never promises to run the Plan and shows both explicit CLI compositions.

Exact artifact behavior

• Default stdout and --output receive the same byte-identical source and
  digest supplied by Make <Plan> emit or capture approved XMD source #722.
• Status, review UI, and planning presentation never contaminate stdout or the
  exclusive output file.
• Stop, exhaustion, failure, cancellation, or final refusal writes no artifact
  and starts no program.
• The output file is created exclusively only after approval and teardown.
• A command continuation or planning session cannot cause the approved program
  to execute implicitly.
• The command has no later program execution to observe or retain.

Acceptance ownership

This story exclusively owns these Quest cases:

• CLI Plan: exact approved bytes reach stdout or the exclusive output file;
  a negative-control program effect never occurs.
• Removed execution option: every --run spelling reports the migration
  above and never aliases or executes.
• Planning-only grammar: only planning and artifact-disposition options are
  accepted; every former execution option refuses before authorship, and no
  accepted option can cause implicit execution.

The successful stdin pipeline belongs to #723; this story owns the Plan
producer and the diagnostic that points to it. #676 owns and later introduces
the planning-only --verbose and --journal options.

Documentation and focused evidence

Rewrite specs/plan-command-spec.md around the single source-artifact outcome.
Update the command/component inventory and command lifecycle in
architecture.md and specs/executable-mdx-spec.md. In particular, remove the
stale inventory claims that <Plan> requires as, emits no source, or that
xmd plan can execute approved source through --run; #722 made <Plan> an
exact text component whose bare form emits and whose as form captures.

Update CLI help, README, and the homepage delivered by PR #719 so they show the
explicit command-line composition:

Plan produces a program.
Run executes a program from the host/CLI.
Composition decides whether and when a planned program runs.

The homepage includes xmd plan | xmd run -. #713 owns the homepage's
<Plan>-followed-by-<Evaluate> example and complete-program evaluation model;
PR #719 remains historical and unchanged.

The Planner handoff inventories the complete revised xmd plan --help output
and every changed user-facing diagnostic as exact text for product approval.
It reuses the source-only command diagnostics already settled in #676 where
they apply and introduces no unreviewed copy during implementation.

Focused feedback evidence:

deno task test \
 packages/cli/tests/plan-cli.test.ts \
 packages/cli/tests/plan-command-document.test.ts \
 packages/cli/tests/plan-component.test.ts \
 packages/cli/tests/syntax-cli.test.ts \
 packages/cli/tests/packaged-document.test.ts

The CLI suite covers every retained and removed option class, exact stdout and
exclusive output, all no-artifact failures, and negative-control non-execution.
After a feedback commit, run deno task test --changed. Delivery proves the npm
binary with scripts/tests/cli-npm-bin.test.ts, the compiled command with
scripts/tests/plan-component-compiled.test.ts after deno task build, and the
required CI checks.

Dependencies

• Make <Plan> emit or capture approved XMD source #722 supplies the exact text-producing <Plan> artifact and its settled
  authorship workflow.
• Add standard-input programs to xmd run #723 makes the prescribed pipe migration executable before --run is
  removed.
• Make xmd plan authorship observable during and after an invocation #676 follows this source-only grammar and later introduces live presentation
  and authorship diagnostics together with the long-form options that select
  them.

Out of scope

• Executing approved source or retaining an execution convenience alias.
• Implementing or reserving inert --verbose and --journal options; Make xmd plan authorship observable during and after an invocation #676
  introduces their long forms with their behavior.
• Complete-program <Evaluate>, owned by Make <Evaluate> evaluate complete XMD programs #713.
• Changing the authorship profile or the <Plan> review policy.