fix(web): stabilize slash autocomplete in new sessions

[jlovec]

Summary

• Fix slash autocomplete race in newly opened sessions where initial remote command fetch may fail
• Add slash-context compensation trigger so recovery does not rely on leaving/re-entering slash mode
• Keep in-flight dedupe and cooldown protections to avoid refetch storms
• Scope slash command cache key by sessionId + agentType
• Add tests for merge filtering and cooldown boundary behavior in useSlashCommands

Test plan

• bun run --cwd web test
• bun run --cwd web typecheck
• Manual: verify first / in a newly opened session can recover after initial fetch failure
• Manual: verify slash suggestions remain stable across session/flavor switches

[github-actions]

[MAJOR] Refetch loop on slash entry after failures

Why this is a problem: HappyComposer triggers onSlashEntry when a fetch settles while still in slash context, and shouldAttemptSlashEntryRefetch returns true whenever an error exists. A failed refetch therefore immediately re-triggers another refetch with no cooldown, which can hammer RPC and keep the UI in a fetch loop when the backend is down.

Suggested fix:

export function shouldAttemptSlashEntryRefetch(
    state: SlashFetchState,
    now: number,
    cooldownMs: number = SLASH_ENTRY_COOLDOWN_MS
): boolean {
    if (state.lastFetchError !== null) {
        return now - state.lastEntryRefetchAt >= cooldownMs
    }
    if (!state.hasFetchedSuccessfully) {
        return true
    }
    return now - state.lastEntryRefetchAt >= cooldownMs
}

[github-actions]

Findings

• [MAJOR] Refetch loop on slash entry after failures can hammer RPC when backend is down (web/src/components/AssistantChat/HappyComposer.tsx:157, web/src/hooks/queries/useSlashCommands.ts:120)

Testing

• Not run (automation)

[jlovec]

按功能拆分后已由新 PR 承接：#255（Web 斜杠自动补全）与 #254（spawn 稳定性修复）。为保持审查范围清晰，此 PR 关闭。