plane: WorkItem state/labels/assignees are objects, not UUIDs

[hstern]

Fixes the second real-deployment regression after v0.1.1's action-tense fix.

The bug

Real Plane CE v1.3.1 serializes nested objects for state, labels, and assignees in the work_item webhook payload:

```json
"state": {"id":"...","name":"Backlog","color":"#60646C","group":"backlog"},
"labels": [{"id":"...","name":"claude:DOCS-25","color":"#FAB287"}],
"assignees": [{"id":"...","display_name":"Alice"}]
```

v0.1.0 / v0.1.1 modelled them as `string` / `[]string`, so every real Plane webhook 400'd on payload decode after the action-tense fix unblocked the verb-mismatch silent-drop:

```
WARN plane: malformed payload: work item: json: cannot unmarshal object into Go struct field WorkItem.state of type string
```

The fix

1. `internal/plane/types.go` — added `StateRef`, `LabelRef`, `AssigneeRef` types and pointed the `WorkItem` fields at them.
2. `internal/sync/planeworkitem.go` — extract UUIDs from `evt.WorkItem.Labels` at the one read site that consumed them (the resolver helper still takes `[]string` UUIDs).
3. `internal/plane/testdata/work_item_{created,updated}.json` — updated to the object form (the wire reality).
4. Regression test `TestParse_RealPlaneWorkItemPayload_PFB24` pins decoding of a captured-from-postgres real Plane payload so the next struct-shape drift fails in CI instead of production.

Docs in this PR

Following the new convention of shipping docs with the fix:

• `CHANGELOG.md` — added in Keep-a-Changelog format with entries for 0.1.0 / 0.1.1 / 0.1.2.
• `README.md` — status header bumped to v0.1.2, image tags updated, "What ships" section reworded to v0.1 (not pinned to a single patch release).
• `internal/plane/README.md` — "Plane API notes" section gains a #0 entry documenting the nested-object wire shape and pointing at the regression test.

Test plan

• lint
• build-image
• e2e-docker (forgejo)
• e2e-docker (gitea)

When merged, tag v0.1.2 — this is a real-deployment-broken bug just like v0.1.1, same patch-release cadence.

🤖 Generated with Claude Code