agent-team: support .agent-team.yml in the consumer repo for per-stage timeouts, test commands, and model overrides

[verkyyi]

Summary

Catalog sets one timeout, one prompt style, one implicit model per stage. Real repos vary: a small personal project wants fast/shallow review; a team repo wants deep review and longer impl budget. Today the only lever is to fork the catalog — which makes upgrades painful (see also sibling issue on installer pre-flight).

Proposal

Catalog agents read .agent-team.yml at repo root (optional) and override their defaults:

# .agent-team.yml
spec:
  model: haiku          # default: auto
plan:
  timeout-minutes: 8
impl:
  timeout-minutes: 20   # overrides catalog default
  test-command: "npm test --workspace=web"
review:
  depth: deep           # triggers branch checkout + full test run
  require-tests: true

Keys are optional and layered on top of catalog defaults. Invalid keys → warning comment on the issue, fall back to defaults.

Acceptance

• Schema documented in catalog/agent-team/README.md.
• Each agent reads the file early and honors recognized keys.
• /install-agent-team offers to create a starter .agent-team.yml with commented defaults.