release: v0.12.3

[bug-ops]

Summary

• Bump version from 0.12.2 to 0.12.3
• Update CHANGELOG.md with release section for v0.12.3
• Update insta snapshots (agent_version, splash screen version string)

Changes included

• fix(agent): restore skill matching and context population on TUI queries (fix(agent): restore skill matching and context population on TUI queries #1017)
  • Fallback to all skills when QdrantSkillMatcher returns empty results
  • Auto-detect context window for ModelOrchestrator in build_provider

Checklist

• Version updated in workspace manifest (all members inherit via version.workspace = true)
• Cargo.lock updated
• CHANGELOG.md has release section with date
• Insta snapshots updated (config_default_snapshot, splash_default)
• cargo +nightly fmt --check passed
• cargo clippy --workspace -- -D warnings passed
• cargo nextest run --workspace --lib --bins passed (2963/2963)

After merge:
git tag v0.12.3 && git push origin v0.12.3