feat: establish MCP-first dashboard transition foundation

[douglasmonsky]

Summary

• establish the Release N dashboard maturity catalog, browser-local experimental preference, lifecycle banners, and conversational-analysis readiness states
• add privacy-safe dashboard evidence targets and canonical thread/call handoff actions for MCP-driven investigations
• retain Call Investigator and Limits as core surfaces while clearly labeling Diagnostics and other unfinished workflows as experimental
• localize the new release shell across all bundled locales and keep packaged dashboard assets deterministic
• add release-candidate browser coverage, governance checks, packaging verification, and the reviewed MCP-first transition design/implementation plan

Product behavior

• Release N keeps the default navigation stable; broader discoverability changes remain deferred to Release N+1
• Call Investigator remains available for deep call analysis
• Limits retains its statistical analysis surface
• Diagnostics remains directly reachable with a highly experimental maturity banner
• MCP readiness and evidence actions provide recovery guidance and safe manual fallbacks when conversational analysis is unavailable

Validation

• npm run dashboard:release-candidate — 11 passed
• /Users/Monsky/.codex/bin/codex-task dashboard-verify --json — PASS
• python scripts/check_release.py — passed
• pytest tests/core/test_dashboard_targets.py -q — 304 passed
• npm --workspace frontend/dashboard run test -- dashboardTargets.test.ts — 33 passed
• git diff --check origin/main...HEAD — passed

Review notes

• The generated dashboard asset changes are intentional and verified against the source build.
• The source-budget baseline is updated as part of the new release governance.
• This PR does not hide additional default navigation items; that product transition remains staged for the next release phase.