Add interactive /changelog promptbar browser

[andrei-hasna]

Summary

• Replace transcript-only /changelog output with a promptbar-adjacent selection browser.
• Parse real release versions and bullets from CHANGELOG.md, with version list -> bullet detail navigation.
• Add Back, Close, and Esc behavior without submitting a normal agent prompt or interrupting a running task.
• Add parser, snapshot, and promptbar interaction regression coverage.

Validation

• UV_CACHE_DIR=/tmp/codewith-uv-cache just fmt from codex-rs: Rust formatter completed; Python SDK/scripts formatter steps failed because DNS prevented fetching ruff from PyPI.
• just test-fast-target /tmp/cw-wt-target -p codex-tui changelog from codex-rs: passed, 8 tests run.
• cargo insta pending-snapshots from codex-rs: no pending snapshots.
• CARGO_TARGET_DIR=/tmp/cw-wt-target just test -p codex-tui from repo root: completed package run but failed 21 pre-existing/environment-sensitive tests; all changelog tests passed in the run.
• CARGO_TARGET_DIR=/tmp/cw-wt-target cargo clippy --fix --tests --allow-dirty -p codex-tui from codex-rs: blocked before linting with failed to bind TCP listener to manage locking / Operation not permitted.
• git diff --cached --check: passed before commit.
• Staged and committed-diff credential scans for common secret patterns returned no matches.

Adversarial review

• Independent reviewer found a parser boundary bug where ## Maintenance Process text could be appended to the last release. Fixed by stopping release parsing at non-release top-level headings and added a regression test.

[andrei-hasna]

Closing as superseded by #211. Replacement #211 consolidates the current TUI/thread-routing stack and explicitly supersedes this PR. Please continue review on #211.