feat(installer): add `codegraph uninstall` command (#313) (#318)

Adds a cross-channel uninstall that removes CodeGraph from every agent it's
configured on (Claude Code, Cursor, Codex CLI, opencode, Hermes). Prompts
global-vs-local up front (no flags required) and reports which providers it
actually hit; --location / --target / --yes supported for non-interactive use.
Removes only what install wrote; leaves the .codegraph/ index to `uninit`.

Also fixes Cursor uninstall leaving an orphaned .cursor/rules/codegraph.mdc
(its description: CodeGraph frontmatter lingered); the dedicated rules file is
now deleted outright while user content outside our markers is preserved.

Validated end-to-end on macOS and Docker Linux (global + local sweeps clean).
Adds 8 tests; full suite 730 passing. Bumps to 0.9.3 with CHANGELOG entry.

Resolves #313.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>