docs: document dispatch-input hardening and pr_number lifecycle

[github-actions]

Follows #69 (fix agent-team manual workflow_dispatch inputs).

What changed

catalog/agent-team/README.md

New pr_number lifecycle section (under The handoff model):

Explains the two-state flow that was implicit in the workflow files but absent from the high-level README:

• First implementation attempt: pr_number is blank → implementer creates a new draft PR
• Reviewer kickback: reviewer passes the existing pr_number back → implementer pushes to the existing branch, not a new one

Without this, users manually retrying a kickback stage couldn't tell whether to supply a pr_number or leave it blank.

Expanded "Input propagation" gotcha (under Limits and gotchas):

Old one-liner only said "fail loudly". New text adds:

• The exact error message agents post (🛑 agent-team: workflow_dispatch inputs were not propagated. Re-dispatch with valid inputs.)
• That unresolved template literals (e.g. $\{\{ github.event.inputs.issue_number }}) are treated the same as missing — the surface-level behavior that PR [codex] fix agent-team manual workflow_dispatch inputs #69 hardened
• That pr_number is optional by design and handled specially

CONTRIBUTING.md

Replaced "no automated test harness" with the actual tiered test strategy.

The old text was incorrect — the repo ships three test tiers and CI runs two of them on every PR. New section provides a quick-reference table and guidance on when to run each tier.

Testing

• No logic changes; documentation only.
• Verified the two edited files render correctly by re-reading them after edits.

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