Clarify AGENTS branch-number rule to remove issue/PR timing ambiguity

[Copilot]

The branch naming guidance in AGENTS.md implied a timing-based choice (“whichever is created first”) while nearby rules required consistent traceability to issue/PR numbers. This updates the wording so number selection is unambiguous and aligned with the rule intent.

• Branch naming guidance

  • Reworded the <number> definition in AGENTS.md to be state-based, not timing-based:
    • use issue number when work is tracked by an issue
    • otherwise use PR number

• Scope

  • Documentation-only change in the branch naming section; no behavior or tooling changes.

- Where `<number>` is either an issue number or a PR number — whichever is created first.
+ Where `<number>` is the issue number when work is tracked by an issue; otherwise use the PR number.