docs(agents): point triage-labels at the canonical umbrella copy

[RonenMars]

The triage vocabulary was canonical by accident in this repo.
The label meanings, the additive rule and the two near-collisions were all written here, while every other convention of the same kind lives in the umbrella — which is the failure threadbase/docs/issue-tracker.md exists to prevent:

A convention that exists in two places becomes two conventions — the first divergence is always a pointer sentence, and by the time anyone notices, the rules have drifted too.

threadbase-streamer adopting the same five labels on 2026-08-20 is what turned a second copy from theoretical into imminent.

What changes

The vocabulary moves to threadbase/docs/triage-labels.md — meanings, colours, the additive rule, the question/needs-info and wontfix collisions, and a compliance query.

What stays here is only what is true of this repo: that all five labels exist, and when each was created.
issue-tracker.md is updated to point at the canonical file alongside the local one.

No label changes, and nothing an existing skill references by name is removed.

Reference

• Parent analysis: P2: skills are duplicated across repos with nothing keeping them in sync #796
• Umbrella side: docs: make the umbrella canonical for triage labels and integration-branch lifecycle threadbase#7
• Streamer's matching pointer docs: docs(agents): add issue-tracker and triage-label pointers [skip-ci] threadbase-streamer#662

Depends on RonenMars/threadbase#7 landing first, since this file links to a document it creates.

Documentation only.