fix(registry): mirror repo layout in cache so cross-workflow refs resolve

[Jason Robert (jrob5756)]

Bug

The registry cache stored each workflow under
<base>/<registry>/<workflow_name>/<sha[:12]>/<filename>, isolating each
workflow in its own subtree. Repo-relative references between workflows in the
same registry repo broke at resolve time:

sdd-plan/plan.yaml # references ../document-review/workflow.yaml
document-review/workflow.yaml

The .. from <cache>/<registry>/sdd-plan/<sha>/plan.yaml resolved to
<cache>/<registry>/sdd-plan/document-review/workflow.yaml — a path that never
existed. Users had to manually copy files into the expected slots as a
workaround.

Fix

Cache layout

Workflows from the same registry+SHA now share a per-SHA root that mirrors the
source repository:

<base>/<registry>/<sha[:12]>/<repo_path> # mirrored repo files
<base>/<registry>/_meta/<sha[:12]>/source.json # cache metadata
<base>/<registry>/_meta/<sha[:12]>/index.yaml # cached registry index
<base>/<registry>/_meta/<sha[:12]>/<workflow>.complete # readiness sentinel

Adhoc:

<base>/_adhoc/<owner>/<repo>/<sha[:12]>/<repo_path>
<base>/_adhoc/<owner>/<repo>/_meta/<sha[:12]>/...

Metadata lives outside the SHA tree so it can never collide with real repo
paths (e.g. a repo's own .conductor/ directory).

Safety + correctness

• Per-workflow readiness sentinel written last so concurrent or
  interrupted fetches never expose a partially populated workflow.
• Atomic per-file os.replace() from a staging dir under _meta/<sha>/,
  intra-filesystem so renames are atomic. Files in the SHA mirror are
  content-addressed by the immutable SHA; concurrent promotions of identical
  content are idempotent.
• _safe_repo_path() rejects .. segments, absolute paths (POSIX +
  Windows), NUL bytes, and empty paths from any index/sibling entry.
• _resolve_within() defense-in-depth ensures resolved targets stay under
  the SHA root after disk resolution.
• source.json carries cache_layout_version, registry_type, source,
  and full_sha; cache hits require all four to match the current registry
  entry. Stale metadata triggers a re-fetch.
• The registry index is cached on disk (_meta/<sha>/index.yaml) so cache
  hits avoid a network round-trip.

Auto-fetch sub-workflows from same registry

When a sub-workflow ref like ../document-review/workflow.yaml resolves to a
path inside a registry SHA cache but the file isn't yet present,
_resolve_subworkflow_path calls a new auto_fetch_relative_workflow() which:

1. Detects the (registry, sha) from the candidate path.
2. Validates the cached metadata (layout version, registry type, SHA prefix
   match, valid GitHub source).
3. Looks up a workflow in the cached index by repo-relative path.
4. Fetches it, populating the shared SHA mirror.

The hook is gated to candidates that look like a file path (separators or
.yaml/.yml extension) and not a registry ref (no @), so registry-style
refs continue through the existing resolver path.

Reserved registry names

add_registry() now rejects names containing /, \, the empty string, or
the reserved _adhoc / _meta namespaces, preventing configurations that
would silently corrupt cache-layout detection.

Backward compatibility

Old per-workflow caches are simply ignored — never read by the new code. They
become wasted disk that conductor registry clear removes. No automatic
migration; users re-fetch on next use.

Out of scope

Cross-directory !file tag references (e.g.
instructions: !file ../other-workflow/prompt.txt) are unchanged. The loader
resolves !file during YAML load before _resolve_subworkflow_path runs;
supporting that case requires hooking the loader and tracking registry context
there. Same-directory !file refs continue to work because siblings are still
fetched alongside the workflow.

Testing

• Full suite: 2633 passed, 11 skipped.
• New cache tests: layout, sentinel-based hit detection, cached-index reuse,
  stale-metadata re-fetch, path-traversal rejection, cross-workflow
  auto-fetch, find_registry_cache_location, metadata-mismatch rejection.
• New end-to-end test (TestCrossWorkflowRegistryRef) reproduces the original
  bug with a parent workflow that references a sibling via
  ../other/workflow.yaml and verifies the auto-fetch hook populates the
  cache and runs the sub-workflow.
• make lint ✅ — make typecheck ✅ (one pre-existing unrelated warning)