Document bug/enhancement as category labels in the triage vocabulary

[vk0eppel]

What to build

The issue forms added in #199 auto-apply bug and enhancement labels, but docs/agents/triage-labels.md only documents the five status labels (needs-triage, needs-info, ready-for-agent, ready-for-human, wontfix). Both axes of the #194–#200 code review independently flagged this drift: the templates apply labels the vocabulary doc doesn't list, so the doc is no longer authoritative.

Resolve it by documenting bug and enhancement as category labels (what an issue is about) that sit alongside the status labels (where it is in triage) — not by dropping them from the forms, since a category label is useful.

Acceptance criteria

• docs/agents/triage-labels.md lists bug and enhancement, framed as category labels distinct from the status/triage-role labels, so the doc covers every label the issue forms apply.
• The distinction between category labels (bug/enhancement) and status labels (needs-triage → needs-info/ready-for-agent/ready-for-human/wontfix) is clear enough that a reader knows an issue normally carries one of each.
• No change to the issue forms or the labels themselves — this is a docs-only reconciliation.

Blocked by

None — can start immediately.

[vk0eppel]

Done in #202 (merged to main as cb727dc). GitHub only auto-closed the first issue in the PR's comma-separated Closes list, hence the manual close.