Fix Codex adapter size budget validation

[Mathews-Tom]

Summary

Fixes the Package All CI failure where scripts/generate_adapters.py --validate rejected the generated Codex root AGENTS.md for exceeding the 32 KiB budget.

This branch updates the Codex adapter index generation and adds focused regression coverage for the budget contract.

Changes

• Tightens truncate_description() so max_chars includes the appended ellipsis instead of allowing truncated strings to exceed the requested limit.
• Adds CodexAdapter.ROOT_DESCRIPTION_MAX_CHARS and applies it consistently to root index entries for rules, agents, commands, and skills.
• Reduces generated Codex root AGENTS.md size from the failing 32,798 bytes to 23,347 bytes, 71% of the 32 KiB budget.
• Adds tests/test_generate_adapters.py coverage that generates the Codex adapter from the real package set and asserts the root file stays within the configured size budget.
• Adds a focused truncation-limit test so future changes cannot silently reintroduce oversized root descriptions.

Validation

• uv run python scripts/generate_manifest.py && git diff --exit-code manifest.yaml && uv run python scripts/generate_adapters.py --validate
• uv run pytest tests/test_generate_adapters.py
• git diff --check main..HEAD

Notes

• Repo-wide uv run pytest passed locally before this branch was pushed: 410 tests passed.
• Repo-wide ruff check, repo-wide ruff format --check ., and targeted mypy --strict are not clean in the current checkout due to existing unrelated lint, formatting, and typing debt. The failing GitHub Actions job does not run those gates.