- Fix misleading comment in resolved_docs_roots() priority chain
- Add failure logging when per-root index search fails (eprintln)
- Add intent comment for grep fallback (first-match, not cross-root merge)
- Add CWD cross-validation to prevent wrong root selection on name clash
- Harden DocRootEntry.expand_path with canonicalize + blocked path check

- Canonicalize DOCS_ROOT env var and legacy docs_root field for consistency
  with expand_path() in docs_roots entries (security: defense-in-depth)
- Fix misleading doc comment on tool_search_global_multi (Rayon always available)
- Add WalkDir max_depth(5) in tool_list_projects_multi (performance)
- Add ambiguity warning when MCP_PROJECT_NAME matches multiple roots
- Derive Clone on ResolvedProject for multi-root scan
- Update tests to use non-blocked paths and canonicalize-aware assertions

- Use actual query string in grep no-match response instead of empty string
- Log read_dir failures in tool_list_projects_multi for debugging

- Replace Value mutation in tool_list_projects with shared list_projects_impl
  using include_root flag, avoiding serde_json round-trip overhead
- Merge grep fallback results from all roots instead of returning only
  the first root with matches, ensuring comprehensive cross-root coverage
- Parallelize rebuild_index across roots using rayon::par_iter
- Remove unnecessary #[derive(Clone)] from ResolvedProject (move suffices)

- validate DOCS_ROOT and docs_root paths with is_dir() before accepting
- improve tool_search_global_multi error message with root count
- raise WalkDir max_depth from 5 to 10 for deeper doc trees
- document build_index thread safety at parallel rebuild site
- replace manual remove_dir_all with RAII TempDir guard in tests

…h sources, remove duplicate test

- mcp.rs: fix ambiguous_env logic — was checking found.is_some() before
  storing the first match, so ambiguity was never detected; restructured
  loop to store first env match then flag on second match
- config.rs: extract expand_tilde() helper; apply tilde expansion to
  DOCS_ROOT env var and docs_root field (previously only DocRootEntry
  used expand_path() which handled ~/); fixes silent failure on paths
  like '~/.alcove/docs'
- config.rs: remove resolved_docs_roots_legacy_takes_precedence_over_vec
  test (duplicate of the precedence check already in
  resolved_docs_roots_uses_docs_roots_vec)
- config.rs: add resolved_docs_roots_expands_tilde_in_docs_root test
- config.rs: add resolved_docs_roots_blocked_docs_root_skips_to_fallback test

Rebase onto main introduced a stale inline resolve loop (is_global guard
and a second let (docs_root, resolved) block) that was already superseded
by resolve_project_multi().  Remove both dead code sections; cargo check
confirms no remaining compile errors.