AGENTS.md: require version label on new PRs and issues

[cliffhall]

Background

PRs created under the issue-driven workflow have not consistently been labeled with the version they target. PRs #1247, #1249, #1250, #1252, #1255 all landed (or are landing) without a label, which makes filtering by version on the PR list unreliable and obscures which work belongs to which roadmap.

Issues have been getting labeled correctly via gh issue create --label v2, but the PR-create workflow doesn't have an analogous reminder.

Scope

Add a rule to AGENTS.md under "Issue-driven Work Style" stating that new PRs and issues must be labeled according to the branch/board they target:

Branch	Board	Label
main	v1	v1
v1.5/main	v1.5	v1.5
v2/main	v2	v2

Acceptance criteria

• AGENTS.md includes the branch → label mapping under the issue-driven workflow section.
• Wording makes clear the rule applies to both issues and PRs.
• Wording clarifies that the label is set at create time (gh pr create --label v2, gh issue create --label v2), not as a separate follow-up step.

Out of scope

• Backfilling labels on already-merged PRs (handled by hand).
• Adding lint / GitHub Actions enforcement for the rule (separate concern; a doc rule is sufficient for now).