fix(codegraph): harden 0.2.1 onboarding readiness

[fishmingyu]

Summary

Hardens the CodeGraph 0.2.1 onboarding path after the post-merge review of #621. The follow-up makes readiness reflect the executable runtime, current repository languages, persisted artifact bytes, and the read-only graph build contract.

Changes

• Re-detect graph-capable languages on every automatic init and reject chunk-only providers.
• Probe the recorded MCP command's graph runtime and verify BM25/graph artifact fingerprints in status.
• Make dry-run project blockers non-zero and require an existing C/C++ compilation database.
• Write Go SCIP output directly to CodeNib state, consume the Rust read-only option, and bind project preparation into graph artifact identity.
• Add focused regressions and update the durable 0.2.1 product roadmap.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring
• Performance improvement
• Tests

Testing

• Tests pass locally
• Added new tests for the changes
• Focused CodeGraph/CLI/toolchain/SCIP/compiler tests: 364 passed, 4 skipped.
• Unit tier: 5208 passed, 71 skipped, 191 deselected; one exact-main umask fixture was excluded after independent reproduction.
• Integration tier: 36 passed, 18 skipped.
• Fresh wheel with graph,mcp extras: CodeGraph init/status, Codex and Claude registration, MCP explore/dependency calls, Dependency Map, idempotent init, and uninstall passed.
• pre-commit and MkDocs strict passed.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published