feat(miner-discovery-plane): add client-side soft-claim coordination request builder

[JSONbored]

Depends on the discovery-index API contract (sibling Phase 6 issue) for its request shape — file both, but the contract should land first or alongside.

packages/gittensory-miner/lib/claim-ledger.js already implements soft-claims, but its header comment is explicit about the boundary this issue extends past: "The database only lives on this machine; this module never uploads, syncs, or phones home" (claim-ledger.js:8-9), and "sibling miners claiming the same issue are resolved elsewhere via isDuplicateClusterWinnerByClaim from @jsonbored/gittensory-engine (#3355)" (claim-ledger.js:81-83) — that adjudication function resolves collisions after the fact, by observing which PR/comment publicly landed first; it has no way to know about another miner's claim before both have already started work. This issue is the piece that closes that gap: a pure, client-side builder that turns a local claim (already recorded via claimIssue/recordClaim in claim-ledger.js) into the request payload a miner would send to the hosted discovery-index (sibling issue) to softly announce/reserve an issue across the fleet before starting, reducing collisions instead of only detecting them afterward.

Scoped as a "request builder," not a network client: this issue produces a typed, pure function that reads the local claim-ledger shape and returns a request payload matching the discovery-index contract — it does not need to perform the actual HTTP call (that's for whichever issue wires the hosted plane's client into the miner runtime, likely downstream of #4250 actually existing).

Deliverables

• A pure buildSoftClaimRequest(claim, context)-style function (in @jsonbored/gittensory-engine, consistent with the discovery-index contract's location) that maps a local claim record's shape (packages/gittensory-miner/lib/claim-ledger.js's rowToClaim output, :59-68: repoFullName, issueNumber, claimedAt, status, note) onto the discovery-index contract's soft-claim request shape.
• No network call inside this function — pure input/output, unit-testable without a server.
• Handle the local ledger's three claim statuses (active, released, expired, claim-ledger.js:12) explicitly — only an active claim should ever produce an outbound "claim" request; released/expired claims should either be rejected by the builder or produce an explicit "release" request variant, whichever the discovery-index contract defines.
• Unit tests under test/unit/, following test/unit/miner-claim-ledger.test.ts's existing fixtures for claim shapes.

References

• packages/gittensory-miner/lib/claim-ledger.js:6-10 (header comment on the never-phones-home boundary this issue extends past), :12 (CLAIM_STATUSES), :59-68 (rowToClaim, the local claim shape to build the request from), :81-83 (the isDuplicateClusterWinnerByClaim/Doc comment explicitly noting this table is a LOCAL bookkeeping primitive only and does NOT itself decide who "wins" a contested issue — that adjudication (reusing isDuplicateClusterWinnerByClaim from @jsonbored/gittensory-engine) is out of scope for this foundation-phase issue. #3355 reference for the after-the-fact adjudication this complements).
• Sibling Phase 6 issue: feat(miner-discovery-plane): define the public-data-only discovery-index API contract — the request shape this builder targets; hard dependency.
• feat(discovery-plane): stand up the hosted discovery-index server #4250 (feat(discovery-plane): stand up the hosted discovery-index server) — "Cross-fleet soft-claim dedup logic (server-side half of the client's soft-claim request builder)" is listed there as a deliverable, i.e. feat(discovery-plane): stand up the hosted discovery-index server #4250 already names this exact client-side piece as its counterpart.

[linear-code]

JSO-3289