perf: batch ingest paths through per-page write transactions

[steipete]

Every ingest path except the provider ran one BEGIN IMMEDIATE transaction per message (~10 statements each, serialized through MaxOpenConns(1)). Benchmarking showed tx-per-page is ~3.4x faster. This converts the remaining paths to accumulate a page of messages into store.WriteBatch and commit per page, following the provider idiom:

• slackapi: history pages and thread-reply pages batch per API page; thread syncs run after the page commits, and thread-skip/channel-skip sync-state bookkeeping stays outside the batch.
• slackmcp: channel messages flush every 500; summary.Messages now comes from WriteBatchResult.MessagesWritten (identical semantics). Thread parents are batched separately from replies so the reply count stays exact.
• slackdesktop: redux states and drafts batch per state/pass; legacy-draft deletions run after the batch commit.
• cli/import: batches 500 messages per flush; validateImportMessageKeys replaces per-message point queries with a chunked bulk IN-list lookup (499/chunk, matching the store preflight precedent), preserving the exact error text.

New additive store flag MessageWrite.SkipWorkspaceCollision turns cross-workspace message collisions into clean in-batch skips. This is transactionally safe: in the batch flow a collision only surfaces after a zero-row upsert, before any writes for that message. WriteBatchResult.CollisionsSkipped reports the skipped keys so the Slack API sync replays each through skipMessageCollision, preserving the per-message warning logs #131 introduced — and a collided parent's thread is not queued for thread sync, matching #131's skip semantics. Desktop paths keep their historical silent skip. Covered by tests in internal/store/batch_test.go (skip-and-commit and default-abort cases).

Intentional semantic shift: a mid-page write failure now rolls back the whole page instead of leaving earlier messages committed — the intended atomicity of page-sized transactions.

Proof: go build ./..., go vet ./..., go test ./..., go test -race ./... all green on top of 10e287b.

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: perf: batch ingest paths through per-page write transactions This is item 1/1 in the current shard. Shard 0/1.

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