feat: add config/local/ overlay for persistent custom configurations

[bookandlover]

Problem

ccb update downloads the latest release and replaces the entire install directory, including config templates (config/claude-md-ccb.md,
config/agents-md-ccb.md, config/clinerules-ccb.md). Users who customize role assignments or review rules lose their changes on every update.

Solution

Introduce a config/local/ directory that acts as a persistent overlay layer:

1. Preservation: copy_project() saves and restores config/local/ during directory replacement
2. Template resolution: New resolve_config_template() helper checks for local override before falling back to upstream default
3. CLI management: New ccb config subcommand (init/edit/show/reset)
4. Documentation: docs/custom-config.md

Usage

ccb config init # Copy templates to config/local/
ccb config edit claude-md-ccb.md # Edit role assignments
ccb reinstall # Apply changes
ccb config show # Check active overrides
ccb config reset # Revert to upstream

Files changed

┌───────────────────────┬──────────────────────────────────────────────────────────────────────────────────────────────────┐
│ File │ Change │
├───────────────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────┤
│ install.sh │ Preserve/restore config/local/; add resolve_config_template(); update 3 config install functions │
├───────────────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────┤
│ ccb │ Add cmd_config() with init/edit/show/reset │
├───────────────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────┤
│ config/local/.gitkeep │ Placeholder directory │
├───────────────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────┤
│ docs/custom-config.md │ Usage documentation │
├───────────────────────┼──────────────────────────────────────────────────────────────────────────────────────────────────┤
│ .gitignore │ Ignore user files, track .gitkeep │
└───────────────────────┴──────────────────────────────────────────────────────────────────────────────────────────────────┘

Design decisions

• Full-file override keeps implementation simple and predictable
• config/local/ inside install dir aligns with existing project layout
• Backward compatible: No changes when config/local/ is empty

Test plan

• ccb config init creates local overrides from upstream templates
• ccb config show correctly identifies active overrides
• ccb reinstall uses local overrides when present
• ccb update preserves config/local/ directory
• ccb config reset removes overrides and reverts to defaults
• No behavioral change when config/local/ is empty

[bookandlover]

CI Failure Analysis

The CI failure is not caused by this PR. The single failing test test_per_session_worker_pool_reuses_same_key also fails on the upstream
main branch (verified locally).

Root cause: _NoopThread in the test fixture passes a list as _started, but Python 3.13's threading.Thread.is_alive() expects _started to be
a threading.Event with an is_set() method.

lib/worker_pool.py:66: in get_or_create
if worker is not None and not worker.is_alive():
threading.py:1143: in is_alive
return self._started.is_set() and not self._handle.is_done()
E AttributeError: 'list' object has no attribute 'is_set'

Local verification:

• main branch: 2 passed, 1 failed (same test)
• This PR branch: 107 passed, 1 failed (same test)

This is a pre-existing issue unrelated to the config/local overlay changes.