Redirect unsupported search scans to find

[Widthdom]

Summary

• Replace the generic search --regex / search --all removal hint with a shell-safe, display-only cdidx find alternative when the invocation maps exactly.
• Return typed JSON recovery fields (argv, POSIX sh, PowerShell, blockers, non-equivalent options, and explicit non-execution markers).
• Reject lossy or invalid alternatives, including incompatible output modes, consumed option-shaped values, byte-budget overflow, invalid snippet limits, and oversized queries.

Root cause

search intentionally preserves indexed-query semantics and does not support scan-only flags, but its usage error did not translate compatible invocations to the scan-oriented find command or explain why a safe translation was impossible.

Impact

Users and automation receive an actionable alternative without changing search semantics or executing another command. Requests that cannot be represented exactly return a reason and no alternative command.

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore (0 warnings, 0 errors)
• dotnet build CodeIndex.sln -c Debug -p:UseSharedCompilation=false --no-restore (0 warnings, 0 errors)
• Issue-focused tests: 10 passed on net8.0 and 10 passed on net9.0
• Full net8.0 suite: 11,036 passed, 0 failed, 7 skipped
• Full net9.0 suite: 10,561 passed, 1 unrelated load-sensitive MCP test failed, 420 skipped; the isolated rerun passed
• Changelog fragment validation passed
• CLI smoke tests covered human, JSON, shell-sensitive and option-shaped queries, structured count output, unmappable requests, and JSON byte limits
• cdidx status --check --json: index/workspace/HEAD matched with no failed checks
• Codex adversarial review: two rounds completed; all five P2 findings were addressed

Documentation and changelog

• Updated the English and Japanese sections in README.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md.
• Added changelog.d/unreleased/4906.fixed.md.

Follow-up

None.

Fixes #4906