feat: continue conversation history for ticketless Slack threads

[m-mizutani]

Summary

Ticketless Slack threads (an @warren mention on a thread that has not been escalated into a ticket) previously lost their AI working memory between turns — the agent appeared to "forget" everything on each new mention.

The infrastructure for cross-turn continuation already existed: every ticketless thread is assigned a deterministic Session ID (slack_ticketless_<hash(channel,thread)>) by ResolveSlackSession, and session history is stored keyed by SessionID (v1/sessions/<id>/history.json), independent of any ticket. The only thing preventing continuation was an if !ticketless guard on both the load and save paths.

This PR removes those guards so that working memory is persisted and restored per thread regardless of ticket association.

Changes

• pkg/usecase/chat.go — buildChatContext now loads session history whenever a Session was resolved (if sess != nil), not only for ticketed threads. This is strategy-agnostic, so both aster and bluebell benefit.
• pkg/usecase/chat/bluebell/bluebell.go — the two terminal saveSessionHistory calls now run for ticketless threads too (saveSessionHistory is keyed by SessionID and is a no-op when no Session was resolved).
• pkg/usecase/chat/aster/aster.go — same change for the default strategy.
• saveLatestHistory (the ticketID-keyed latest.json crash-recovery snapshot) remains gated on !ticketless: it requires a ticket and is not used for cross-turn continuation.

Scope / impact

• No struct, field, schema, or signature changes — only branch conditions and comments.
• Behavior changes only for the "ticketless + resolved Session" combination, i.e. Slack ticketless threads. Web/CLI require tickets (or run with no Session) and are unaffected.
• Ticketed threads are completely unchanged.
• No migration needed. Pre-existing ticketless threads have no saved history, so their first mention after this change is a fresh start; subsequent turns continue.

Tests

• TestBluebellChat_TicketlessSavesSessionHistory / TestAsterChat_TicketlessSavesSessionHistory — ticketless + Session persists history under the SessionID.
• TestChatFromSlack_TicketlessContinuesConversation — end-to-end via ChatFromSlack: the deterministic Session is reused across mentions and saved history is restored into chatCtx.History on the next turn.
• go vet, golangci-lint, gosec, and the full go test ./... suite all pass.

Docs

• doc/strategy/README.md gains a "Conversation Memory" section documenting SessionID-keyed working memory and ticketless continuation.

[gemini-code-assist]

Code Review

This pull request enables conversation memory persistence for ticketless Slack threads. Previously, session history was only loaded and saved for ticketed threads. Now, working memory (gollem.History) is persisted per Session (keyed by SessionID) for both ticketed and ticketless threads, allowing conversation context to carry over between turns even when no ticket exists. This change is implemented across the aster and bluebell chat strategies, and corresponding unit tests have been added to verify the behavior. I have no feedback to provide.

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