[Refactor] Generalize Fast parent event delivery

[daniel-lxs]

What

• make FastAgentConversation and FastAgentParent shared, validated contracts in @roomote/types
• replace Slack-specific parent payload fields with a provider-neutral conversation identity
• separate parent-event coordination from the Slack delivery adapter
• rename delivery state from slackPosted to the provider-neutral replyPosted
• keep Slack-only structured input rendering behind an explicit surface guard

Why

The Fast core can now treat Slack and Discord as adapters, but delegated tasks still persisted their parent as Slack coordinates and the lifecycle coordinator contained Slack delivery inline. This stacked refactor removes that coupling and leaves an explicit surface adapter seam for adding Discord lifecycle delivery without changing the orchestration loop again.

This is stacked on #1452. Discord parent stamping and delivery are intentionally the next follow-up; this PR makes that addition isolated instead of mixing it into the contract migration.

Validation

• @roomote/types: 55 files, 849 tests
• @roomote/cloud-agents: 95 files, 769 tests
• @roomote/slack: 38 files, 402 tests
• @roomote/sdk: 99 files, 996 tests
• affected Worker suites: 2 files, 24 tests
• pnpm lint:fast
• pnpm check-types:fast
• pnpm knip

The full Worker suite reached 1,747/1,748 tests; the unrelated tail-file-path.test.ts symlink assertion failed outside this diff. The affected Worker suites pass.

[roomote-community]

No code issues found. See task

• packages/types/src/fast-agent.ts:19 Preserve parsing of legacy persisted Fast parent payloads so existing child runs retain lifecycle delivery, input handling, and snapshot-resume inheritance. Not applicable: the superseded representation exists only in this unmerged PR stack.

_{Reviewed ac34c3b}