feat(web): add safe persisted session deletion

[testikun]

Problem

Related to #347. Web Workbench can archive Sessions but cannot safely remove a persisted non-active Session. Deletion must not be confused with archive metadata removal, and the active Session must never be deleted.

Value

Adds a bounded, auditable persistence-management primitive for Session retention while keeping Pi JSONL files authoritative and preventing accidental active-session loss.

Approach

• Add an authenticated DELETE /api/sessions?path=... endpoint.
• Resolve the target from the canonical Pi Session projection and require a .jsonl file inside the configured Web Session directory.
• Reject the active Session with an explicit 409 SESSION_CONFLICT response.
• Remove the persisted file first, then clean Web-owned archived and ungrouped derived indexes.
• Publish a session_deleted event for connected clients.
• Keep native confirmation/UI, archive browsing, historical editing, and fork semantics out of this backend-only slice; those remain separate lifecycle work.

Validation

• Focused Node tests: 34 passed, 0 failed.
• Full Node/Vitest suite: 1339 passed, 1 skipped, 0 failed; Vitest 30 passed.
• biome format / biome lint --error-on-warnings: passed.
• tsc --noEmit: passed.
• Config-contract, discipline-ledger, and Web syntax checks: passed.
• Ablation: removing the typed deletion-conflict error caused the active-session API regression to return 500 instead of the required 409; the error boundary was restored.
• bun is not installed in this environment, so the equivalent repository scripts were run with the bundled Node 24 executable and local Biome/Vitest binaries.

Impact

• User-visible behavior: backend deletion API and deletion event; no UI changes in this PR.
• Model-visible context/tools: none.
• Runtime/lifecycle: active Session deletion is rejected; non-active persisted files are removed.
• Persisted data: targeted JSONL file is deleted and Web-owned index entries are cleaned.
• Compatibility/risk: endpoint is authenticated and exact-target bounded; native confirmation remains a required follow-up before exposing this mutation in UI.