fix(mcp): discover client roots for home-launched servers

[sunerpy]

Summary

• Add server-initiated MCP roots/list support when serve --mcp starts with no usable project root.
• Kiro-style launches from $HOME now stay safe initially (no daemon/watch/catch-up against home), then adopt the first indexed root returned by the client.
• Split workspace-root parsing/adoption into codegraph-mcp::roots and document the home-launch recovery path.

Verification

• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all --check
• oxfmt --check docs/mcp.md docs/cli.md
• bash scripts/guardrail.sh
• Manual stdio QA: fake HOME launch emits roots/list, consumes client roots response for this indexed repo, and tools/list returns populated tools including codegraph_explore.

Notes

• v0.15.4 home-root guard remains: daemon, watcher, and catch-up are never started against $HOME.
• Clients that advertise MCP roots can now recover the real workspace without per-project --path.