feat(proto): canonical Issue/PullRequest type family + SubscribeEvents variant (SEA-1727 S1a)

[seal-agent]

SEA-1727 — S1a proto: canonical Issue/PullRequest type family + SubscribeEvents variant (read contract)

Proto half of S1 of the frozen compass-issue-model record (#1018, Active), reconciled by the two Active sibling amendments (compass-server-ownership-layer-amendment: single canonical wire family, no raw forge shape on the wire; compass-attribution-simplification: attribution is a plain display fact). The forge/tracker integration contract for the dogfood cut. Gates S1b (compass-server, SEA-1728) and S1c (compass-ui, SEA-1729) on the read path.

Proto-only. Authored by compass-repo (sole proto writer).

Scope note — write path deferred (Matt, 2026-08-04)

This PR ships the read contract only. Matt ruled the issue-state write model is agent-primary: the UI is read-only for issue state, agents drive the board lifecycle, and users change issue state via the issue tracker, not Compass. That amends the frozen record's DL-031 (board was human-primary) and DL-032 (tracker was a downstream projection) and is pending a design amendment before any write surface ships. So the UpdateIssueState RPC that the frozen record specified is NOT in this PR — it is held for the amendment (which will place an agent-facing lifecycle path on AgentGateway, not CompassService, since egress-sealed agents cannot reach CompassService). The type family + read path here are needed under any write model, so they land now to unblock S1b/S1c.

What's in it

proto/compass/v1/compass.proto:

• Canonical type family: Issue (18 fields), PullRequest (15), IssueState enum (UNSPECIFIED..ARCHIVED, 0-8), AgentAttribution, ForgeProvider/ForgeRef, ChecksSummary/Check, ChangedStats, TrackerRef, Review/ReviewThread/Comment.
• Issue issue = 16 variant on the SubscribeEventsResponse oneof — the read/delivery surface; canonical objects reach the UI as snapshot + live fan-out off the server projection (design.md:349-450). Only Issue (PRs nest in Issue.prs = 17).
• AgentAttribution is the slimmed shape per compass-attribution-simplification (DL-094): { string agent_handle = 1; reserved 2, 3; reserved "owner_handle", "verified"; }.
• Field numbers/names/types byte-faithful to the frozen design.md as amended; ledger ids cited in comments (DL-032/033/050/051/069/091/094).

Verification

• compass-proto:ci 5-green: buf lint (STANDARD; ENUM_ZERO_VALUE_SUFFIX armed — both enums carry _UNSPECIFIED = 0), buf breaking (additive-only), gen drift (regen matches committed), gen-fence (canonical types are public — correctly unfenced).
• compass-go:test all packages OK (no classifier tripwire — no new RPC on the gated services).
• compass-client:ci (22 pass + typecheck), compass-agent:typecheck green.
• Review loop (skill://review): clean pass on the type family, 0 high / 0 medium.

Sequencing

S1a (this, read contract) → S1b (SEA-1728, server ingestion/projection over the read types) → S1c (SEA-1729, UI reads the board). The agent-primary write path follows in a design amendment.

Refs SEA-1727.

[linear-code]

SEA-1727