Adds GitLab Quick Action autocomplete

[Nat1anWasTaken]

What changed

• Replaces the standalone Quick Action composer with GitLab-aware autocomplete inside Markdown descriptions and comments.
• Loads permission-aware commands from GitLab and searches members, labels, work items, merge requests, epics, milestones, iterations, snippets, branches, and projects.
• Supports keyboard navigation, caret-positioned suggestion menus, quoted references, and create-card descriptions.
• Moves advanced quick-create fields into an apply/cancel dialog.
• Returns Quick Action summaries for command-only comments and refreshes the board after GitLab applies them.
• Scopes background mutations to the fields stored with each operation, preventing unrelated edits from re-executing description commands.
• Updates API contracts, documentation, styling, tests, and visual audit screenshots.

Why

Quick Actions need to execute with GitLab’s permission and parsing rules rather than a partial client-side implementation. Native description and comment syntax keeps the workflow consistent with GitLab while autocomplete makes commands and references discoverable. Field-scoped durable operations also prevent assignee or date updates from replaying stale commands embedded in a description.

Validation

• Added coverage for autocomplete command matching, reference providers, quoted labels, caret popover placement, actor-scoped GitLab requests, command-only comment responses, sparse mutations, and HTTP contracts.
• Updated board interaction and visual audit coverage for the new quick-create dialog, search semantics, native pickers, and description autocomplete.
• Exact test commands and manual flows run: not provided.
• Updated UI screenshots under docs/assets/ for the changed layouts and dialogs.

Contract and operations

• Adds authenticated endpoints for permission-aware Quick Action catalogs and reference suggestions.
• Changes comment creation responses to include the created comment, execution status, and GitLab summary; command-only comments may return 202 Accepted with no cached comment.
• No database migration or configuration change is required.
• Quick Action execution remains actor-scoped through existing OAuth credentials; successful command-only comments trigger a board refresh.
• No new rollout dependency is required. Rolling back removes the new endpoints and frontend autocomplete, while existing description/comment behavior must be restored alongside the response contract.