feat(work-items): detect whether the tracker stores status in the Work status field or in labels #11123

Refs: #7577

Agent Brief

Type: Feature
Summary: The work-item tracker reports where an item's status is stored: the Work status organization issue field when the tracker offers it, else labels. This is the parent's status-field-adapter slice.

Current behavior:

• plugins/work-items/tools/work-item-tracker/CONTRACT.md (around :235-255) defines set-fields and get-field-options over organization issue fields, read from GET /orgs/{org}/issue-fields. Only the GitHub adapter implements them.
• No verb or capability tells a caller whether status is a field or labels; every status reader and writer uses labels.
• work-items: probe org issue field for work status #7600 recorded the probe: the org field is Work status (single-select; options needs-triage, needs-reporter-info, needs-owner-decision, needs-spec, agent-ready), and a token with issues: write can set it.

Desired behavior:

• The tracker exposes status storage detection (verb or capability name left to the implementer, documented in CONTRACT.md).
• The github adapter reports field storage when the Work status organization issue field exists, read from the same issue-fields listing set-fields uses. It reports labels when the field is absent, or when the listing cannot be read (user-owned repository, no read:org).
• The local-markdown adapter always reports labels.

Done when

• CONTRACT.md documents the detection and its output.
• Conformance or tracker test cases, fixtures only, cover: github with the field present (field), github with it absent (labels), github with the listing unreadable (labels), and local-markdown (labels).
• The tracker test suite and the conformance suite pass for github and local-markdown.
• scripts/check-changelog-fragments.sh passes with a work-items fragment.

Out of scope:

• The agent-ready label migration and moving work-resolve-issue.yml's trigger to issues: field_added (the parent's status-migration slice).
• Creating or changing organization issue fields (owner stop).