Adopt madder digest-pinning (FDR-0008) for repo-id resolution

[friedenberg]

Context

madder's FDR-0008 (config digest pins) lets an id carry a name@<markl-id>
suffix that is AssertEqual'd at resolve time against the target config's
self-check digest — disambiguating same-named stores cryptographically and
position-independently, and detecting tampering.

dodder should adopt the same for -repo_id resolution, as a complement to
the positional dot-depth (..name) multi-dot resolution (#281) — keep both;
digest-pinning is the cryptographic disambiguator on top of the positional one.
This directly helps the nested same-named repo case (cf. #281, #283): notes@<digest>
pins the exact intended notes repo regardless of nesting/position.

Current state

• scoped_id.Set already parses name@<digest> (madder, FDR-0008 Phase 2)
  and exposes GetDigest() / HasDigest() / WithDigest().
• dodder's repo_id package drops the digest entirely: EffectiveName,
  EffectiveId, and CheckSupported never read it. So dodder show -repo_id notes@<digest> parses but the digest is ignored.

Scope (to design)

1. Pin source. Decide what per-repo digest the suffix verifies against —
   candidates: the content-addressed immutable-config digest (get_konfig_sha)
   or the repo public-key in config-immutable. Possibly adopt FDR-0008
   Phase-1-style self-check on the dodder repo config-seed first.
2. Resolution. Thread the digest through repo_id resolution: locate
   candidate repos (the existing scope/ancestor walk), then AssertEqual the
   pinned digest against each candidate's digest; error (with expected-vs-actual)
   on mismatch / no match.
3. Surface. -repo_id name@<digest> on the CLI + MCP; optionally emit the
   digest-bearing canonical form in info-repo repos / persisted references.
4. Relationship to FDR-0019: multi-dot (..name) — operate path ignores cwd-depth (drop the cwd-depth>0 reject) #281/Nested same-named cwd repo can't commit: genesis !md type blob not found in its own store #283. Complement dot-depth; consider whether
   digest-pinned resolution also sidesteps the nested-commit store ambiguity in
   Nested same-named cwd repo can't commit: genesis !md type blob not found in its own store #283.

Deferred to the end of the current FDR-0019 task sequence; filed for tracking.

Environment: dodder smart-pine branch; madder pin go v0.3.43-…-d3de5838721b
(FDR-0008 Phase 2 present in scoped_id).