fix(mcp): scope Kiro MCP to project path and guard home-root indexing

[sunerpy]

Summary

Kiro launches the stdio MCP server from $HOME and its initialize handshake carries no rootUri/workspaceFolders and does not advertise capabilities.roots (confirmed by capturing Kiro's real handshake). A bare global serve --mcp therefore could not discover the project, degraded to home safe mode, and previously risked indexing the whole home tree.

This makes Kiro project-scoped and hardens indexing against too-broad roots:

• installer(kiro): inject --path like Cursor — local install pins the concrete project root, global install pins ${workspaceFolder} for per-workspace expansion — so every Kiro window serves its own project.
• cli(init/index): refuse a too-broad root ($HOME or filesystem root /) instead of building a home-wide index that would peg a CPU on a home-launched serve.
• mcp(serve): on late client-root adoption, stop the home direct loop and proxy the current stdio session into the adopted project's shared daemon (run_until_adoption); effective for clients that advertise MCP roots.
• docs: document Kiro project-level install and the init/index home-root guard.

Verification

• make ci green locally (fmt + clippy -D warnings + full workspace test + guardrail); pre-push gate green.
• New regression tests: Kiro local/global --path injection; guard_indexable_root rejects $HOME and /, allows nested project; run_until_adoption preserves the post-adoption stream for proxy handoff.
• Real installer QA: install --target=kiro --local writes --path <project>; --global writes --path ${workspaceFolder}; init $HOME is refused with no .codegraph created; init in a real project still builds the db.
• Cargo.lock diff is version-only churn (no new dependencies).