docs: add changelog entry for v0.5.0

[olivrg]

Description

Release notes for v0.5.0 — the adapter-readiness slice merged since v0.4.0:

• Added: sideband governance API (Sideband decision API: framework-neutral governance endpoints for hook-based adapters #12, experimental), context-aware policy primitives (Policy engine: context-aware primitives #13), dashboard rendering of adapter-origin tool calls (Dashboard: render adapter-origin tool calls #16).
• Security: patched form-data to >= 4.0.6 (GHSA-hmw2-7cc7-3qxx).

Also fixes the stale [Unreleased] compare link and adds the [0.5.0] reference.
No code changes — changelog only. The v0.5.0 tag (which drives publish) is pushed after this merges.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (no functional changes)
• Documentation
• CI / build / tooling

Packages Affected

• packages/proxy
• packages/dashboard
• packages/python-sdk
• Root config / monorepo tooling
• docs/
• examples/

Checklist

• I have read CONTRIBUTING.md
• My code follows the existing style (ESLint + Prettier pass)
• TypeScript strict mode — no any types or @ts-ignore without justification
• I have added or updated tests for my changes
• All CI checks pass (pnpm secrets:scan, pnpm docs:check:ci, pnpm audit --audit-level=high, pnpm build, pnpm lint, pnpm format:check, pnpm typecheck, pnpm test)
• I have updated documentation if this changes user-facing behavior
• Commit messages follow Conventional Commits (e.g. feat:, fix:, docs:)
• format:check / docs:check:ci pass

How to Test

Additional Context