feat(linear): Agent Interaction essentials for SDK-compatible sessions

[liam-germain]

Summary

• Align @emulators/linear with the current production Linear Agent Interaction contract and @linear/sdk v89.
• Model agent activity content as the production GraphQL union, accept the production nested input shape, and validate type-specific content, signals, IDs, context metadata, plans, and unique external URLs.
• Implement the activity lifecycle: newer ephemeral thoughts and actions archive the previous active ephemeral activity, while activity connections support production-style filters, archive visibility, ordering, and Relay pagination.
• Keep the actor boundary explicit: the seeded lin_test_agent OAuth app token owns agent sessions and activities, while human prompts use the separate internal-style agentActivityCreatePrompt mutation with lin_test_admin.
• Queue prompts while an agent turn is active, deliver the oldest prompt after a terminal response, and reopen the session for the next turn without emitting premature webhook events.
• Emit production-shaped AgentSessionEvent payloads for created and prompted, including OAuth client identity, structured child objects, exact optional fields, and Linear-Timestamp.
• Build escaped prompt context from the issue, team, labels, project, directive comment, and prior comments instead of interpolating raw user content.
• Remove the legacy flat activity input, legacy session aliases, and other compatibility-only fields.
• Update the README, package docs, docs site, Linear skill, and CLI help surface.

The emulator remains a focused Linear subset. Rich guidance storage, repository suggestions, and the rest of Linear's unrelated product schema remain outside this PR.

Test plan

• pnpm --filter @emulators/linear test (37 tests, including end-to-end @linear/sdk v89 coverage)
• pnpm --filter @emulators/linear type-check
• pnpm --filter @emulators/linear lint
• pnpm --filter @emulators/linear build
• pnpm type-check
• pnpm lint
• pnpm format:check
• Hosted CI green on this PR

The full monorepo test aggregate was also attempted locally. It is blocked in this sandbox by an unrelated Vercel Blob test trying to listen on 0.0.0.0 and receiving EPERM; the Linear suite passes independently.

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