feat: select between Codex and Claude

[justin13888]

Summary

• bump agent-text to 0.1.1 and enable its Codex adapter
• add --agent codex|claude with automatic Codex-first executable detection
• use Luna/Terra model tiers for Codex while preserving Haiku/Sonnet for Claude
• update CLI, library, README, and Homebrew documentation for both agents

Validation

• cargo test -p git-aicommit agent::tests — passed
• cargo test -p git-aicommit cli::tests — passed
• cargo fmt --all -- --check — passed
• cargo clippy --workspace --all-targets -- -D warnings — passed
• cargo test --workspace — passed (54 unit tests and 1 doctest)
• cargo build --release — passed
• documented aicommit-core dependency/process boundary checks — passed
• help and version with no agent on PATH — passed
• ruby -c .github/homebrew/git-aicommit.rb — unavailable (ruby is not installed in this environment)

Risks and rollout

• automatic selection checks executable presence on PATH; authentication, compatibility, and generation failures remain explicit and do not silently fall back
• no live model generation was invoked during validation