feat: allow planning CLI tools (gh, glab) in plan mode

[MattPark]

Issue for this PR

Closes#23984

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Plan mode's system prompt blocks all bash commands with absolutist language ("ZERO exceptions", "commands may ONLY read/inspect"). This prevents legitimate planning activities like creating GitHub/GitLab issues via gh and glab CLI tools, forcing users into build mode prematurely.

The bash tool isn't hard-denied in plan mode's permission config — it inherits "allow" from defaults. The refusal is caused entirely by the system prompt.

This PR makes three changes:

1. plan.txt — Scopes restrictions to "source code modifications" instead of "all commands." Adds an "Allowed planning activities" section listing issue/ticket management, PR/MR creation, documentation artifacts, and project queries.

2. prompt.ts — Same scoping for the experimental plan prompt.

3. agent.ts — Adds bash: { "gh *": "allow", "glab *": "allow" } to the plan agent permission config so the intent is explicit at the permission layer too.

Source code edit restrictions remain unchanged (edit: { "*": "deny" } in permissions + prompt language).

How did you verify your code works?

Reviewed the three changed files for correctness. The prompt changes only affect planning-related tool usage — source code protections (hard edit: { "*": "deny" } permission + prompt restrictions on sed/tee/echo/cat) are preserved. The gh * / glab * bash rules use the existing granular permission pattern matching system documented in the permissions docs.

Screenshots / recordings

N/A — prompt and permission config changes, no UI.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[github-actions]

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