feat(installer): add Kiro CLI/IDE target (#385) (#473)

`codegraph install` now detects and configures Kiro alongside the
existing seven agents. Writes `mcpServers.codegraph` to
`~/.kiro/settings/mcp.json` (global) or `./.kiro/settings/mcp.json`
(local), plus a dedicated `~/.kiro/steering/codegraph.md` /
`./.kiro/steering/codegraph.md` instruction file — Kiro's steering
system loads every `*.md` file in `steering/` as agent context, so a
dedicated file is the natural surface (no marker-based merging needed).

Sibling MCP servers in `mcp.json` and unrelated steering files
(`product.md`, `tech.md`, etc.) are preserved across install and
uninstall. Validated end-to-end on macOS, Linux (Docker node:22-bookworm
arm64), and Windows 11 (Parallels VM, Node 24): full installer-targets
suite passes (132 tests) on all three platforms, and live install /
idempotent re-run / uninstall round-trip works as expected.

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