chore(release): prepare v0.5.0

[waylandzhang]

Summary

Release prep for v0.5.0. Version bump + changelog + a build-hygiene fix. No code/runtime changes beyond .dockerignore.

This does not publish anything. The release images + GitHub release are produced by release.yml when a v0.5.0 tag is pushed (after this merges to main).

Changes

• CHANGELOG.md — [0.5.0] section covering work since v0.4.1: LiteLLM provider, xAI x_search, MiniMax-M3, Anthropic 1M context, content-blocks passthrough, attachment MIME classification, Python SDK 0.7.0, cache-aware quota/billing (1h/5m TTL, cache_aware_total_tokens), gpt-5.1 priority, optional playwright-service profile, the Missing runtime files #173 proto fix, 429 rate-limiter fix, cache-TTL uniformity, desktop libs.
• scripts/install.sh — default SHANNON_VERSION v0.4.0 → v0.5.0.
• deploy/compose/docker-compose.release.yml — example header → v0.5.0.
• docs/rocky-linux-quickstart.md — note that committed proto runtime files (Missing runtime files #173) supersede the manual grpc_tools.protoc workaround; regenerate via make proto-local (pinned protobuf 5.x), never an arbitrary protoc version.
• .dockerignore — exclude generated **/*.pb.go so a stale dev-tree copy can't clobber buf-generated protos during Go image builds (Python grpc_gen _pb2.py are committed and intentionally not matched).

Local verification (before this PR)

• All 5 images build clean from a fresh main worktree (mirrors CI).
• llm-service image: protobuf 5.29.2, grpc_gen imports OK, cache-token fields present at runtime (Missing runtime files #173 proven).
• Full release-compose stack came up healthy (8 containers); gateway /health, orchestrator /readiness, and /api/v1/tools all green (keyless infra smoke).
• .dockerignore hardening validated — orchestrator builds even from a dirty tree with stale .pb.go.

Release steps after merge (separate, explicit)

1. Tag v0.5.0 on merged main → triggers release.yml (builds/pushes 5 images + latest, creates GitHub release).
2. Watch the release workflow.
3. Installer smoke from the published tag.
4. Close Missing runtime files #173.