Allow deleting the #general chat thread; enforce at-least-one-thread invariant

[ptone]

Summary

The #general thread in native chat cannot be deleted — three layers of guards prevent it (store, handler, UI). Users who want to reorganize their threads around project-specific topics are stuck with the default #general thread forever. The request is to make #general deletable while enforcing that at least one thread always exists, and falling back to the first thread in the list when no #general is present.

Current state

#General deletion guards (three layers)

1. Store layer — DeleteTopic in both SQLite (webchannel_store.go:874-884) and Postgres (webchannel_store_postgres.go:505-514) checks is_general and returns "cannot delete #general topic".
2. Handler layer — handleTopicDelete (handlers_chat_v2.go:664-667) surfaces the error as HTTP 400.
3. UI layer — chat-space-rail.ts:1663 hides Delete/Rename menu items for isGeneral threads; handleDeleteThread (line 1245) has an early return guard.

Navigation assumes #general exists

• selectSpaceBySlug (chat.ts:1282-1301) uses threads.find(t => t.isGeneral) with no fallback to threads[0].
• handleCollapsedSpaceClick (chat-space-rail.ts:929-933) — same pattern, no fallback.

Lazy #general re-creation (critical interaction)

ListTopics in both store implementations auto-creates #general if none exists (webchannel_store_postgres.go:453-464, webchannel_store.go:828-843). If #general becomes deletable, this lazy creation would immediately re-create it, defeating the purpose.

No "last thread" guard

Neither store nor handler checks thread count before deletion.

What must change

1. Make #general deletable

• Store: Remove is_general guard in DeleteTopic (both dialects). Replace with a "last remaining thread" count check.
• Handler: Replace the #general error with a "cannot delete the last thread" error.
• UI: Remove !thread.isGeneral guards hiding Delete from the context menu. Show error to user when "last thread" deletion is rejected.

2. Enforce at-least-one-thread invariant

• Store: Before soft-deleting, count active (non-deleted) topics for the project. If count ≤ 1, return error.
• Handler: Surface as 400 validation error.

3. Navigation fallback when no #general

• selectSpaceBySlug (chat.ts:1282-1301): Fallback to threads[0] when no isGeneral thread found.
• handleCollapsedSpaceClick (chat-space-rail.ts:929-933): Same fallback.
• No sort changes needed — existing sort already produces stable order.

4. Remove lazy #general re-creation

• Remove auto-create from ListTopics (webchannel_store_postgres.go:453-464, webchannel_store.go:828-843). Keep EnsureGeneralTopic only on project creation path (handlers_projects_core.go:592-625).

5. Decide on #general rename

• Currently blocked (handleTopicPatch:571-574). If #general is deletable, renaming should also be allowed. Remove the rename guard.

Edge cases

• Messages in deleted threads: No FK constraints — messages survive soft-delete but become inaccessible via UI (history endpoint returns 404 for deleted topics). Acceptable for v1.
• Read state orphaning: webchat_read_state rows keyed by topic UUID become orphaned. Harmless but accumulate.
• Conversation entity: Topic deletion does not delete the linked conversations row. May need cleanup.

Sizing

Small — changes are well-localized: 2 store files, 1 handler file, 2 frontend files, plus test updates. Conditional pivot to Medium if rename support, data migration, or conversation-entity cleanup are included.

Files

• pkg/hub/webchannel_store_postgres.go:453-464,503-523,546-595 — Postgres store (lazy creation, delete guard, ensure)
• pkg/hub/webchannel_store.go:828-843,872-893 — SQLite store (lazy creation, delete guard)
• pkg/hub/handlers_chat_v2.go:524-676 — Handler (patch/delete guards)
• web/src/components/shared/chat/chat-space-rail.ts:1220,1245,1663 — UI guards
• web/src/components/pages/chat.ts:1260-1303 — Navigation fallback
• Investigation doc: scratchpad/projects/roadmap/inv-chat-thread-deletion.md