feat(miner): exclude discover candidates assigned to the repo owner

[JSONbored]

Summary

• Closes AMS contribution-profile: exclude candidates assigned to the repo owner in discover #7040, a narrow follow-up to AMS contribution-profile: wire eligibility filtering into discover #6798/feat(miner): wire ContributionProfile eligibility filtering into discover #7024. That PR candidly scoped out assignee-exclusion — the third of the three exclusion rules the epic (epic: AMS per-repo contribution-profile discovery (labels, docs, agent-facing eligibility signals) #6793) originally called for — because threading assignees through the fan-out was "a genuine second shared-file change." This does that threading and adds the check.
• Extends RawCandidateIssue with assignees — already present in the same GitHub list/search payload discover already fetches, no extra request.
• Adds a new excluded_assignee reason to contribution-profile-filter.js for a candidate assigned to its own repo's owner login.

Design note

Unlike the three label-based rules, this check is structural and always-on — per the schema (ContributionAssigneeRuntimeCheck), assignee-exclusion is deliberately not a profile field, so it runs ahead of (and independent of) the existing eligibilityLabels.confidence === "explicit" safe-default gate the label rules are conditioned on. It applies regardless of whether the repo has a trustworthy ContributionProfile at all — tested explicitly (a candidate assigned to the repo owner is excluded even with no profile in the map).

Test plan

• npm run typecheck — clean.
• npm run test:coverage (full, unsharded) — 979 files / 18322 tests passed, 0 failures.
• New tests: 7 in contribution-profile-filter.test.ts (excludes owner-assigned, case-insensitive match, doesn't exclude other assignees, applies with no profile, applies with low-confidence profile, tolerates missing owner/assignees, ignores malformed assignee entries) + 2 integration tests in miner-discover-cli.test.ts (excluded from ranking/enqueueing end-to-end, applies without a profile).
• All 12 pre-existing contribution-profile-filter.test.ts tests pass unchanged (the new check is purely additive; candidates without owner/assignees are never affected).

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.75%. Comparing base (a752b8f) to head (995686f).
⚠️ Report is 19 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7042   +/-   ##
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  Coverage   93.74%   93.75%           
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  Files         692      692           
  Lines       68740    68752   +12     
  Branches    18768    18768           
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+ Hits        64443    64455   +12     
  Misses       3302     3302           
  Partials      995      995           

Flag	Coverage Δ
shard-1	43.38% <0.00%> (-0.42%)	⬇️
shard-2	36.85% <75.00%> (+<0.01%)	⬆️
shard-3	33.16% <66.66%> (-0.20%)	⬇️
shard-4	34.39% <8.33%> (+0.49%)	⬆️
shard-5	31.63% <33.33%> (-0.58%)	⬇️
shard-6	46.07% <8.33%> (+0.30%)	⬆️

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Files with missing lines	Coverage Δ
.../loopover-miner/lib/contribution-profile-filter.js	100.00% <100.00%> (ø)
packages/loopover-miner/lib/opportunity-fanout.js	98.27% <100.00%> (+0.03%)	⬆️