fix: drop unused siblings section from mymir_context working depth

[FrkAk]

Summary

mymir_context depth='working' had a ## Siblings section that called fetchSiblingTasks(projectId, taskId) — a query that returns every other task in the project, no filter. For a 50-task project, every working-depth call dumped 50 lines of - TASK-N "title" (status) into the agent's context. The section was also semantically redundant: ## Connected Tasks already renders all 1-hop neighbors (both depends_on and relates_to, both directions, with edge notes) via getTaskEdgesDetailedTx.

This PR drops the section and its supporting code. ## Connected Tasks is the lone neighbor surface in working depth going forward, which is strictly more useful (edge type + direction + note per neighbor).

• lib/context/_core/working.ts — drop siblings from WorkingContext, drop the fetchSiblingTasks call, drop formatSiblingsSection and its caller, update JSDoc.
• lib/data/task.ts — delete fetchSiblingTasks + SiblingTaskInfo (no other callers in the repo).
• lib/mcp/create-server.ts + lib/graph/tool-handlers.ts — drop "siblings" from the three MCP tool description strings agents see every turn.
• tests/context/working.test.ts — rewrite the test that asserted result.siblings.length === 1 (it ratified the bug) and add a regression-gate test pinning that relates_to 1-hop neighbors continue to land in result.edges.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation

Testing

• Tested locally with bun run dev
• Linting passes (bun run lint)
• Typecheck passes (bun run typecheck)

Notes for reviewer

• Test suite: bun test → 458 pass, 0 fail. The rewritten + new tests in tests/context/working.test.ts carry the behavioral guarantee — the new third test pins that relates_to 1-hop neighbors continue to land in result.edges, which is the basis for removing the Siblings section.
• Final grep sweep run: grep -rn "siblings|fetchSiblingTasks|SiblingTaskInfo" lib/ tests/ app/ returned zero MCP-layer references (UI / unrelated test prose / db raw helper exclusions documented in the plan).
• The UI (components/workspace/detail/DetailView.tsx, BundlePreview.tsx, RelationshipsSection.tsx) uses the words "siblings" / "neighbors" in its own bundle-preview component with a DIFFERENT semantic (it filters to relates_to 1-hop only). That vocabulary is the UI's contract and is intentionally untouched.
• The tests/context/working.test.ts rewrite is two changes: the existing first test was renamed and shrunk to focus on the positive RLS ancestor check (its real job — the sibling assertion was bolted on); a new third test pins the relates_to-via-edges guarantee.
• No schema changes. No migration. No data-layer behavior change beyond deleting an unused function.