feat(index): add --hidden flag for dot-prefixed files

[peterkc]

Summary

Add --hidden CLI flag to include hidden (dot-prefixed) files and directories in indexing and search operations.

Use case: Users want to index configuration directories like .claude/, .config/, or .github/ that are hidden by default.

Changes

• Add --hidden flag to CLI (ck-cli/src/main.rs)
• Add show_hidden field to FileCollectionOptions (ck-core)
• Add hidden field to SearchOptions (ck-core)
• Make WalkBuilder.hidden() conditional on config (ck-index)
• Wire flag through MCP server, TUI, and engine
• Add integration test for --hidden functionality
• Update CHANGELOG.md

Behavior

Command	Hidden files
ck --index .	Excluded (default)
ck --hidden --index .	Included
ck --hidden --sem "query" .	Searched

The flag defaults to false to maintain backward compatibility.

Test plan

• cargo test --workspace passes
• Manual verification with .hidden-test/ directory
• Integration test: test_hidden_flag_includes_hidden_files
• --hidden + --no-ignore combination works correctly

[runonthespot]

Hi @peterkc 👋

This PR is well-implemented (clean show_hidden plumbing through ck-core → ck-index → ck-cli → MCP/TUI, integration test included, proper default-false for backward compat). It's been sitting in draft since January.

Heads-up: main's CI was broken from late January through today and has just been fixed (#110 + #116). Your branch is now testable.

Would you like to:

1. Mark this ready for review (and rebase on current main) — I'll review and merge.
2. Hand it off — say the word and one of us can pick it up and finish.
3. Close it if you don't need it anymore.

No pressure, just nudging since it's a small, useful feature.