docs: freeze changelog for v0.5.6

[caigee-cmd]

What

Prepare the next patch release (0.5.6) by writing the bilingual release notes for the only user-facing change since v0.5.5 and moving them out of ## Unreleased.

The change carried by this release is the namespace-tool / reasoning-replay work merged in #186:

• Expand Codex/Desktop type: "namespace" tool wrappers into plain function tools (namespace__name) for the Responses adapter, Trae, and WorkBuddy; drop hosted shells (type: "mcp" / web_search)
• Replay Responses reasoning items onto the following assistant message or function call as reasoning_content (WorkBuddy thinking mode)
• Accept content-block arrays in function_call_output and lift input_image blocks into a following user message

Checks

• python3 scripts/release-notes.py validate — pass
• python3 scripts/release-notes.py self-test — pass
• python3 scripts/release-notes.py extract-for-release v0.5.6 — emits the 3 EN / 3 中文 bullets
• go test ./..., go vet ./... — pass (docs-only change)

Release

Merging this does not publish anything. After merge, run the Release workflow from main with bump = patch to tag v0.5.6 and publish the image.

Notes

• The version number is not stored in the tree; it comes from the git tag, so this PR only touches CHANGELOG.md.
• Separate audit findings not included here (out of scope, no code change requested): deploy/README.md says both "three previous stable releases" and "five most recent snapshots" for rollback; docs/PRIVATE_DEPLOYMENT.md is a local, gitignored runbook still pinned to v0.5.1.