fix(runner): read the model catalog from --codex-bin

[GauranshMathur]

Why

load_catalog always ran codex from PATH, ignoring --codex-bin and CODEX_BIN. Those are the documented fix when codex is not on PATH: the troubleshooting reference's answer to codex: command not found and the exec skill's preflight both point to them. Yet such a run failed with "Is codex installed and on PATH?" before codex was ever started. When a codex was on PATH, the catalog could also describe a different build from the one the run used.

I found this while checking ADR 0001 (#18), which is about how the catalog is read. It's a small fix nobody raised, so per AGENTS.md there is no issue.

Changes

• load_catalog(codex_bin="codex") runs debug models with the given binary.
• main reads the catalog from args.codex_bin. The catalog_loader seam stays for tests and defaults to None.
• The error names the binary it tried and points at --codex-bin and CODEX_BIN.

Verification

• New tests/test_catalog.py, run with PATH emptied and a stand-in codex reachable only by its path:
  • the catalog is read from the named binary
  • a full run with --codex-bin succeeds when codex is not on PATH
  • an unreadable catalog's error names the binary and the fix
• The end-to-end test failed on main with exactly the reported error, CatalogUnavailable: Could not read the codex model catalog. Is codex installed and on PATH?, and passes with this change.
• pytest: 50 passed
• claude plugin validate . --strict passes

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https://claude.ai/code/session_017PZwFoejyVocqgdjxs9ech

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