chore(release): v0.16.0

[hashedone]

Manual release bump since release-plz is blocked by the org Actions quota.

What

• Workspace version: 0.15.0 -> 0.16.0
• tracevault-core dependency in cli/server: 0.15 -> 0.16
• Changelogs updated for tracevault-core, tracevault-cli, tracevault-server

Highlights

• agent-policies: new CLI command + MCP tool + dashboard preview, all
  backed by a server-side renderer
• branches: repo column + per-repo filter
• sessions: tool / user / file-changes filters and token breakdown
• policies: removed both scope; "Window" renamed to "Validation"
• mcp: tracevault-mcp server with ask_tracevault + agent_policies tools

Next steps after merge

1. git tag v0.16.0 && git push origin v0.16.0
2. cargo publish -p tracevault-core
3. cargo publish -p tracevault-cli
4. Once Actions are back, the release-binaries workflow will pick up the tag and update the Homebrew formula.