Share the leaf filter in work and the parent walk in impact, and inline two skeleton checks

[Deicyde]

Follow-up to #92 and #136. No behaviour change: 3 files, +18/-42, one commit per item.

• Unfinished leaves (work.py, list_ready_work). The checks for missing article IDs and missing article revisions each repeated the same dispatchable, non-Mathlib, unproved filter over the runtime nodes. Both now read one list, and raise the same errors in the same order.
• _ancestors (impact.py). _owners and _descends_from each walked a constant's parent chain with the same cycle guard. The walk is now one generator. _owners still returns at the first parent an article names, and name in _ancestors(...) stops at the first match, as _descends_from did.
• _report_entry (skeleton.py). _report_kind and _report_withheld each had one caller. Their checks now sit where each field is read, with the same messages.

Noticed, not changed:

• No test reaches the "invalid declaration kind" error in _report_entry. The withheld-flag error is covered.

Overlaps with open PRs:

• Keep work impact working beside Lean files whose names are not identifiers #182 and Allow proof imports with measured impact #183 edit impact.py and skeleton.py, Deploy Pages only after autoform verify passes, and refuse a lean: name defined twice #140 and Bind statement: formalized to a recorded statement_hash #141 edit all three files, and Add verified statement-review bundles #75 edits skeleton.py.
• Each merges with this branch without adding a conflict, and a pyflakes and duplicate-definition check of the merged files finds nothing new.
• Add verified statement-review bundles #75 conflicts with main, and Deploy Pages only after autoform verify passes, and refuse a lean: name defined twice #140 and Bind statement: formalized to a recorded statement_hash #141 target open-statements-revision, which conflicts with main.

Validation at exact head ac4a5148: ruff check autoform_cli servers tests is clean. tests/test_work.py (30), tests/test_impact.py (73), tests/test_contract.py (2) and tests/test_cli.py (5) pass. Without Lean on PATH, tests/test_skeleton.py gives 144 passed and 28 skipped. CI's real-Lean job runs the other 28. With Lean installed, all 172 passed locally with this skeleton.py change applied to main.