Specification: Explain a pane's screen classification

Created: 2026-10-09. Status: clarified.

Screen rules (specs 027, 028) change a pane's state from data the user cannot see. herdr answers "why is this agent blocked?" with agent explain. This adds the same read-only answer as pane.explain, so users and agents can debug a rule without guessing, per docs/14 directives 3 and 4.

Acceptance

1. pane.explain {pane_id} returns the pane's kind, live, hooked, the rule source (user, bundled or none), classifies (true when the screen tick would act on this pane: live, unhooked, with rules), every rule of that source as {id, state, priority, region, matched} against the current title and visible screen, and the winning matched rule {id, state, priority, index} or null (index is its position in rules, since rule ids may repeat across manifests).
2. It never changes pane state and emits no event. Unknown pane → NO_SUCH_PANE; bad params → BAD_PARAMS.
3. signaltty pane explain ID prints the same data (--json raw) with one line per rule, the winner marked.
4. Real-PTY integration tests cover a bundled match, a user override, a hooked pane (classifies false) and an unknown pane; docs/08 gets the row.

Scope and clarification

Explains screen rules only; hook and process detection are reported as the hooked flag and kind, not traced. No region text preview: pane.read already returns the screen. No unresolved requirements remain.