chore(release): prepare v0.1.12

[chaliy]

Summary

• Bump version to 0.1.12
• Update CHANGELOG.md with 4 PRs since v0.1.11

What's Changed

• feat(fs): restore SearchCapable/SearchProvider traits (feat(fs): restore SearchCapable/SearchProvider traits #779)
• refactor(builtins): adopt read_text_file helper across 17 builtins (refactor(builtins): adopt read_text_file helper across 17 builtins #778)
• chore(skills): move repo skills under .agents (chore(skills): move repo skills under .agents #777)
• refactor(builtins): share lossy text file reads (refactor(builtins): share lossy text file reads #775)

Test plan

• cargo fmt --check passes
• cargo clippy --all-targets --all-features -- -D warnings passes
• cargo test --all-features passes
• CI green on PR
• Merge triggers release.yml to create GitHub Release v0.1.12