Boards: add an Incoming status to both boards, adopt the v2 Priority field, and document the triage rubric

[cliffhall]

Follow-on from the issue-backlog triage in #1872.

Problem

Two gaps surfaced while triaging the 154 unlabeled open issues.

1. There is no board state for "filed but not yet triaged." AGENTS.md defines four Status options — Todo, In Progress, In Review, Done — and describes Todo as "approved-but-not-started work." So a newly filed issue has nowhere to sit that distinguishes nobody has looked at this yet from a maintainer approved it and it is queued. During the sweep this mattered: the 22 issues left open as v2 are awaiting maintainer review, and parking them in Todo would have falsely asserted they were approved.

2. The board's Priority field exists but is unused and undocumented. Board #28 carries a Priority single-select left over from the project template. Zero of the 264 items had it set, AGENTS.md never mentions it, and its options (P0/P1/P2) match no vocabulary used anywhere in the repo. Meanwhile there is no written rule for how to decide that an issue is urgent — priority has been implicit in whoever was triaging.

The work

1. Add an Incoming Status option to board Add tab and approval flow for server -> client sampling #28, ordered ahead of Todo, for newly filed and untriaged issues. Todo keeps its meaning: maintainer-approved and ready for work.
2. Adopt the Priority field with the vocabulary Urgent / High / Medium / Low.
3. Document a scoring rubric so priority is derived rather than asserted: severity (1-5) + urgency/staleness (1-5) + signal bonuses, summed and banded into the four levels.
4. Document all of it in AGENTS.md — the new Status option in the board recipes table, the Priority field and option IDs alongside the Status ones, and the rubric as its own section.

Notes

• Both board mutations are done and verified. updateProjectV2Field full-replaces the option list, so each ran with every existing option's id echoed back; the Status mutation was verified card-by-card afterward (all 264 statuses unchanged, all four prior option IDs preserved). See the hazard callout in AGENTS.md.
• The Priority re-option was risk-free in ordering terms because no card had a value set.
• The board recipes and their IDs are deliberately not mirrored into .github/copilot-instructions.md — that file is scoped to what a reviewer cites against a diff.