Harden conversation send and export controls

[andrei-hasna]

Summary

• block sensitive content and routing metadata before Conversation messages are persisted across CLI, MCP, interactive, and HTTP /v1 send paths
• redact legacy sensitive content in read/export/digest/search, notification previews, channel bridge blurbs, summaries, and review/control output
• add synthetic regression coverage for notification, summary, MCP, interactive, and HTTP API paths

Validation

• bun test src/lib/channel-notifications.test.ts src/lib/summary.test.ts src/mcp/index.test.ts src/mcp/channel.test.ts src/server/api.test.ts --reporter=dots: 108 pass, 0 fail
• bun run typecheck: pass
• bun run build: pass
• bun test --reporter=dots: 1007 pass, 0 fail
• git diff --cached --check: pass before commit
• gitleaks protect --staged --no-banner --redact: no leaks before commit
• gitleaks detect --source . --no-banner --redact --log-opts=origin/main..HEAD: no leaks after commit

Review

• adversarial re-review found no blocking findings after P1 fixes for pinned summaries and HTTP /v1 routing fields

No live data redaction or credential rotation was performed by this PR.