fix(devspec_locate): support case-insensitive docs/ and naming conventions

[bakeb7j0]

Summary

Fixes devspec_locate to handle multiple project conventions:

• Both lowercase docs/ and capitalized Docs/ directory layouts
• Multiple file naming conventions (<project>-devspec.md, devspec-<project>.md, dev-spec.md)

Changes

• Modified handlers/devspec_locate.ts to accept case-insensitive directory and multiple naming patterns
• Updated tests/devspec_locate.test.ts to cover all naming variants

Tests

• ✅ All 2075 tests pass
• ✅ New tests cover lowercase/capitalized docs, both naming patterns

Closes #276