Add PR triage skills: status labeller + Mon/Wed/Fri digest

.agents/skills/_shared/pr_triage/README.md

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+# Shared PR triage core
+
+`labels.py` is the single deterministic source of truth for CausalPy PR triage.
+It fetches open PRs via `gh`, computes a set of orthogonal facts per PR
+(lifecycle, conflict, CI, review, decision-needed, risk, idle-band,
+author-class), and derives one `next_action` per PR via an explicit precedence
+order.
+
+Two skills consume it, and consume the *same* function so they can never
+disagree:
+
+- `pr-status-labeller` — writes `status:*` labels to GitHub.
+- `pr-digest` — produces the read-only Mon/Wed/Fri Discord digest.
+
+No LLM judgment lives here. All configuration (maintainer/labs handles, idle
+thresholds, precedence order, managed label set) is at the top of `labels.py`;
+edit it there and nowhere else.
+
+Run standalone to inspect classification:
+
+```bash
+python labels.py                # human-readable table
+python labels.py --json         # structured output
+```