feat: add parsec ci to check CI/CD pipeline status

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

feat: add parsec merge to merge PRs from terminal

Add `parsec diff` command that shows the diff between a worktree branch
and its base branch using merge-base for accurate comparison. Supports
--stat, --name-only, and --json output modes.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Add support for stacked/dependent PRs via `--on` flag and `parsec stack` command:

- `parsec start <ticket> --on <parent>` creates a worktree branched from
  the parent ticket's branch instead of main/master
- `parsec stack` displays the dependency graph as a tree
- `parsec stack --sync` rebases the entire chain in topological order
- `parsec ship` automatically uses the correct base branch for stacked PRs
- Added `parent_ticket` field to Workspace struct (backward-compatible via serde default)

Closes #29

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Allow `parsec start` to attach to an existing local or remote branch
instead of always creating a new one. If the branch exists only on
remote, it is fetched and tracked automatically.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

- Add "What is parsec?" section to README for AI citation
- Add schema.org JSON-LD structured data (SoftwareApplication + FAQPage)
- Optimize Cargo.toml description and keywords for crates.io
- Update GitHub repo description and topics

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

New in 0.2.4:
- parsec ci — CI/CD pipeline status with watch mode
- parsec merge — merge PRs from terminal with CI wait
- parsec diff — view worktree changes vs base branch
- parsec stack — stacked PR dependency tracking
- parsec start --branch — attach to existing branches
- SEO and AI search discoverability improvements

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>