Config file for CLI flag defaults

[BrooksFlannery]

Problem

The same flags get retyped on nearly every invocation, e.g.

onton --repo . --backend codex
onton --gameplan <path> --backend codex --mode auto

There's no way to say "for me, --backend codex --mode auto are always the defaults."

Proposal

Extend the existing user config at ~/.config/onton/<owner>/<repo>/config.json (already loaded in bin/main.ml:4103 via lib_core/repo_config.ml) with an optional defaults block that supplies fallback values for CLI flags:

{
  "defaults": {
    "backend": "codex",
    "mode": "auto",
    "model": "...",
    "pollInterval": 30,
    "maxConcurrency": 5,
    "mainBranch": "main"
  },
  "routing": { ... },
  "reviewBackends": [ ... ]
}

Resolution precedence:

CLI flag > env var > per-repo user config > hardcoded default

Optionally also load a user-global ~/.config/onton/config.json between the per-repo file and the hardcoded defaults, so "codex everywhere" works without duplicating the file per repo.

Out of scope (for now)

• Repo-scoped .onton/config.json checked into the repo for team-wide defaults. Worth doing eventually, but skip for v1 — start with user-scoped.
• --gameplan — per-invocation, would invite stale defaults.
• --token — already env-driven via GITHUB_TOKEN; don't tempt people to commit secrets.

Implementation sketch

1. Add a defaults record to lib_core/repo_config.ml (all fields optional).
2. In bin/main.ml flag resolution, change each Arg.value (... ~default:hardcoded) to consult the loaded config first.
3. Tests: a property that CLI flags still win over config; that missing config keys fall back to the existing hardcoded defaults.

[subsetpark]

Done.