Work scopes: bind candidate content to a named mutable source for live recheck #60

The 0.3.0 companion deliberately distinguishes CANONICAL default-branch
scopes from isolated CANDIDATE artifacts. Candidate subject identity currently
binds an immutable commit but not a mutable branch/MR content-source ref.
Consequently the collector cannot prove that a live candidate branch still
points at the recorded content commit during an issue/research/analysis recheck.
Reusing the old candidate SHA as a network collection argument would otherwise
make a moved branch appear unchanged.

The release now fails closed before any network request for these candidate
content tasks unless the execution client supplies a freshly attested
--inventory from its current checkout/source. Canonical content tasks are
rechecked against the current default-branch SHA. Candidate PR review/merge
tasks already refresh their selected PR's exact head/base and are unaffected.
Candidate snapshots never advance canonical latest.

This is a specific follow-up beneath #54/#56, not another request for a queue
engine. The safety guard has a negative regression:
test_work_candidate_content_cannot_recheck_an_unrecorded_mutable_ref.

Acceptance:

• Introduce explicit, versioned content-source bindings for candidate branch
  refs and MR/PR head refs, including fork source identity where required.
• Resolve that named source through a bounded read-only call before live
  recheck; compare its current immutable SHA to the verified candidate scope.
• A moved/deleted/inaccessible/ref-ambiguous source stays invalidated or UNKNOWN;
  never fall back to a historical SHA and label it current.
• Validate candidate content commit existence/project binding at collection,
  while retaining explicitly attested immutable offline analysis inputs.
• Test branch moves, force pushes, fork MR sources, deleted refs, auth/rate caps,
  wrong project and default-branch changes independently of candidate changes.
• Preserve canonical/candidate store isolation and no mutation/claim authority.