🤖 refactor: dedupe SSH/Coder startup barrier replay logic

[ammar-agent]

Summary

This follow-up to #3133 deduplicates the logic that keeps the SSH/Coder startup barrier consistent during reconnects and workspace switching.

Background

#3133 fixed a concrete bug: startup detail like Checking workspace runtime... could disappear if you switched away from an SSH/Coder workspace and then switched back.

The first cleanup pass in this PR already moved low-level stream-lifecycle and runtime-status helper logic into src/common/types/stream.ts. This update follows through on the next cleanup opportunities nearby:

• make WorkspaceStore use one replay-buffering path for previewing replayed startup/error state before caught-up
• stop re-deriving "is this workspace still working/starting?" from raw aggregator fields in ProjectSidebar
• pull AgentSession's replayed startup/terminal status emission into a named helper

Implementation

• shared stream-lifecycle / runtime-status helper logic now lives in src/common/types/stream.ts
• AgentSession now replays terminal/preparing status through emitReplayStatusSnapshot(...) instead of open-coding that state machine inside emitHistoricalEvents()
• WorkspaceStore now routes pre-caught-up buffering through getBufferedReplayEventBehavior(...), so replayed stream-error, stream-lifecycle, stream-abort, and runtime-status follow one preview/buffer path
• ProjectSidebar now uses replay-aware sidebar state for:
  • archive confirmation warnings
  • active/working checks
  • attention checks
    instead of recomputing startup state from raw aggregator fields
• ProjectSidebar tests now provide getWorkspaceSidebarState() in the mocked workspace store so the shared sidebar-state path is exercised directly
• focused coverage for the shared stream helpers lives in src/common/types/stream.test.ts

Validation

• bun test src/browser/stores/WorkspaceStore.test.ts src/node/services/agentSession.preStreamError.test.ts src/browser/components/ProjectSidebar/ProjectSidebar.test.tsx
• bun test src/common/types/stream.test.ts src/browser/utils/messages/StreamingMessageAggregator.runtimeStatus.test.ts src/node/services/agentSession.preStreamError.test.ts src/browser/stores/WorkspaceStore.test.ts
• make typecheck
• make static-check
• git diff --check

Risks

Low. This is intended to stay behavior-preserving, but it touches the reconnect/workspace-switch startup path in both the backend replay layer and the renderer. The main regression risk would be stale or missing startup barrier state while replay is hydrating.

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[ammar-agent]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

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[ammar-agent]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

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