write CONTRIBUTING.md; dedupe Unreleased; add make verify (T-109)

Three docs sweeps from the consultant batch:

* CONTRIBUTING.md lands as a human-addressed guide — clone, build,
  test, DQR, tickets, commit conventions. README links to it.
* CHANGELOG `[Unreleased]` consolidated to one subsection per kind
  in Keep a Changelog order (Added, Changed, Removed, Fixed,
  Security). Released sections left frozen.
* `make verify` is the no-tests sweep: analyze + format +
  decisions-validate + changelog-gate. Bridges the gap between
  `make analyze` alone and the full `make push-check`.

Item 3 (cut an interim release) deferred — that's the user's call
on timing.

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -45,6 +45,7 @@ Pre-v2.0 (`2.0.0-dev`). Interaction model and panel system landed. The Python Te
 ## Documentation
 
 - [`docs/architecture.md`](docs/architecture.md) — current architecture (read this first).
+- [`CONTRIBUTING.md`](CONTRIBUTING.md) — how to clone, build, test, file tickets, and write D-records.
 - [`docs/initial-plan.md`](docs/initial-plan.md) — historical design doc; preserved as a snapshot of the 2026-04 plan, much of it now superseded.
 - [`governance/decisions/`](governance/decisions/) — confirmed decisions (`D-NNN`), open questions (`Q-NNN`), rejected alternatives (`R-NNN`).
 - [`CLAUDE.md`](CLAUDE.md) — Claude-addressed working notes (guardrails, repo layout).