AMS contribution-profile: design the ContributionProfile schema and caching shape

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Part of #6793. Depends on #6794 (the signal inventory this schema should be grounded in, not designed in the abstract).

Context

AMS needs a concrete data shape to represent what it's learned about a repo's contribution eligibility rules, before any extraction or discover integration can be built against it.

Requirements

• Define a ContributionProfile shape covering (informed by AMS contribution-profile: inventory real-world eligibility signals across a diverse repo sample #6794's actual findings, not assumed upfront):
  • Eligibility label rules: which label(s) mark an issue as contributor-workable (loopover's gittensor:* is one instance, not the only possible shape — a generic repo might use help wanted/good first issue/something else entirely, or nothing).
  • Exclusion label rules: which label(s) mark an issue as maintainer-only / off-limits.
  • Assignee-exclusion rules: e.g. "not assigned to the repo owner."
  • Linked-issue / PR-body requirements: does a PR need to reference a specific issue with a closing keyword.
  • A confidence/completeness indicator: was this profile built from explicit, unambiguous signals, or inferred/partial — this matters for how conservatively discover should treat an uncertain profile.
  • Source provenance: which signal(s) each rule was derived from, for debuggability.
• Decide the profile's storage/caching shape (a new local SQLite store mirroring existing miner stores like policy-doc-cache.sqlite3, keyed by repo, with a sensible TTL/refresh policy — labels and docs both change over time).

Deliverables

• A documented schema (a markdown doc under packages/loopover-miner/docs/ alongside the existing miner-goal-spec.md precedent, plus any TypeScript/JSDoc type definitions needed for the implementation issues that follow).
• No extraction logic or discover wiring in this issue — schema/design only.

Expected Outcome

The extraction-implementation and discover-integration sub-issues have a stable, precise target shape to build against.

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JSO-4644