Support # document references in the plan editor's Ask AI #1556

Summary

Part of #1543 (Track A2). Let users # reference documents and tags in the orchestration plan editor's Ask AI, so a revised plan can use them. The same server authorizer serves the workflow assistant in Phase 3.

User Value

When revising a plan, users often want to say "also check this against #policy-doc". Today they have to leave the editor and start over. The normal composer's # picker already works well, so the plan editor should offer the same thing.

Proposed Behavior

• Picker: reuse the normal composer's # picker (searchContextCandidates() in lib/contextMentions.ts:208-275, DocumentPickerPopover, ContextChips, and keyboard navigation in ComposerEditor.tsx:429-480) through A1's restricted ComposerEditor mode.
• Server: factor a scope-only reference authorizer out of resolve_elicitation_references (functions_orchestration_context.py:346), which today requires an owned conversation. Surfaces without a conversation, such as the workflow editor, accept only personal, group and public documents and tags, never chat attachments. The elicitation path stays exactly as it is.
• Plan editor: # references in Ask AI flow into the plan's seeds through merge_elicitation_context, which is already used for editor-scoped questions (functions_orchestration_plan_editing.py:354-372).
• Not planned: the diagram, chart and image editors edit an artifact, so they don't get # references.

Acceptance Criteria

• A user can # a document in the plan editor's Ask AI, and the revised plan uses it.
• Unauthorized or stale references are rejected with a clear message.
• The orchestration question card behaves exactly as before.
• Functional tests for the scope-only authorizer (including rejection of chat attachments), Playwright coverage of the plan editor picker, release notes and config.py version.

Notes

• Roadmap: §6, Track A, A2. Related gotchas in §8: 22 (ComposerEditor is shared), 23 (reference authorization outside a conversation), 24 (group or public documents bound into a personal workflow).
• Open decision (§9), with the recommendation: a "search all my documents" toggle in the plan editor is a later option, not part of A2.
• Depends on: A1 (Show AI-assist messages immediately, with Cancel and Retry, in a shared assist thread #1552). Phase 3 (Add an AI workflow assistant to the V2 workflow editor #1548) depends on this.