Drop Root and Kind: Projects are git repos found by Scan

[hammadmajid]

Breaking change for v0.3.0. Settled in a grilling session.

Discovery

• A Project is any directory holding a .git entry (dir or file), identified by its absolute path.
• Only cdd scan [dir...] walks the disk. No args walks $HOME. Directories are not remembered.
• The walk goes to any depth and stops descending at a repo — except a scan argument, which is always descended into (and recorded as a Project if it is a repo).
• No symlink following (except scan arguments). Unreadable dirs are skipped silently.
• include_hidden (default false) skips hidden dirs during the walk; .git detection is unaffected.
• exclude is gitignore-style: a pattern containing / matches the absolute path (~ expanded), otherwise the basename. Default [].
• Scan prints only its final summary.

History & Picker

• History stores absolute paths. Relative (pre-v0.3.0) lines are skipped as malformed; re-run cdd scan.
• The Picker lists History only and never walks. A Visit whose <path>/.git is gone is Stale and gives no row. The "never-visited last" rule disappears.
• Row: muted ~/…/parent/ + bold basename; the filter matches the full path. The preview header uses the same form.
• Exact-match: cdd <query> jumps directly when exactly one Project matches by basename or trailing path suffix.
• git.Status states stay; only "not a git repository" copy goes.

Config

• config.toml becomes optional.
• root is removed; a stale root is a hard error explaining the change.
• picker.layout stays, accepts only "list" (the new default); "grouped" is a hard error explaining the change.

Docs

• CONTEXT.md: drop Root and Kind; redefine Project, Scan, Stale Visit, Layout.
• ADR docs/adr/0001-projects-are-git-repos-found-by-scan.md.
• README and demo updated (VHS 0.11.0).

[hammadmajid]

Shipped in PR #58, released as v0.3.0: https://github.com/kryft-dev/cdd/releases/tag/v0.3.0