docs: add config-set-tool change proposal

[avoidwork]

Description

Implements the config-set-tool change: extracts :config set config mutation logic from src/config/loader.js into a dedicated src/config/mutate.js module, and wires it into a new set skill tool.

Commits

• docs: add config-set-tool change proposal with design and specs (a0f7f45)
• feat: extract config mutation into dedicated mutate.js module (aa4e9ee)
• feat: resolve :config set command path parsing and missing setValue wiring (881f5fb)
• feat: add path depth limit of 5, fix test import paths (a2b1ec4)
• feat: update config schema with set skill tool (c38c9ab)
• chore: finalize config-set-tool spec and tidy up old change (6fe4ef0)

Changes

• New: src/config/mutate.js — self-contained mutator module exporting:
  • parseValue(str) — boolean/number/string coercion
  • assignPath(obj, path, value) — dot-path mutation with intermediate object creation and depth limit (5)
  • applyDotPathMutation(config, dotPath, valueStr) — full pipeline (parse → clone → assign → validate)
• New: config-set tool registration via config schema
• Modified: src/config/loader.js — imports applyDotPathMutation from mutate.js, setConfigValue calls applyDotPathMutation then saveConfig
• Updated: tests/unit/config/mutate.test.js — 16 tests for all mutator exports
• Cleaned: tests/unit/config.test.js — removed duplicate mutation engine tests
• Tidied: archived config-set-tool change, removed stale replace-assistant-label-with-name change

Tests

• ~207 tests pass (same count, no regressions)
• 100% coverage maintained

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)

Testing

• npm run test — all tests pass
• npm run coverage — coverage.txt regenerated, 100% coverage maintained
• npm run lint — passes clean

Coverage

• 100% line coverage maintained

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied