fix(core): restore permission-aware subagent guidance

[tlysanhuo]

Issue for this PR

Closes #35238

Type of change

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

What does this PR do?

V2 already rejects disallowed subagent targets at execution, but the model-facing filtered list disappeared when the public session request hook was removed in 761f373.

This adds a location-scoped SubagentGuidance instruction source. It lists only visible, non-primary agents whose effective per-target subagent rule is not deny, and emits instruction updates when that list changes. The existing runtime assertion remains the hard boundary; regression coverage verifies that a denied target creates no child session while a specifically allowed target still runs.

How did you verify your code works?

• Core targeted tests: 8 passed
• Core full suite: 1281 passed, 6 skipped, 0 failed
• Core typecheck
• Prettier check and Effect pattern scan

Screenshots / recordings

Not applicable; no UI change.

Checklist

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

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[tlysanhuo]

The PR already references Closes #35238, and the issue timeline shows the cross-reference. This targets the non-default v2 branch, so GitHub does not populate closingIssuesReferences; the standards workflow consequently adds needs:issue for V2 PRs (the same behavior is visible on other V2-targeted fixes). A maintainer will need to clear the label or adjust the workflow check for non-default release branches.

[tlysanhuo]

@rekram1-node This addresses the remaining model-facing half of #35238 after runtime enforcement landed in #35794. The placement follows the current durable instruction-guidance pattern instead of restoring the removed public request hook; feedback on that V2 direction would be appreciated.