fix(storage): prune redundant intermediate message_update frames

[wyuc]

Fixes #1278.

What

After a message_end is durably appended, prune the intermediate message_update rows of that message's contiguous update run, keeping only the first and last. This materializes at write-time exactly what the read path already discards.

Why

pi streams an assistant message as many message_update events and every frame is persisted carrying the full accumulated message. The read path (readEventsAfterForReplay / client compactReplayEvents) already keeps only the first+last of each contiguous run, message_end carries the full text, and crash/resume never reads message_update (it reconstructs from the entry tree + pi transcript). So intermediate frames are persisted but never consumed — pure write amplification (measured ~74% of rows / ~89% of bytes in one deployment).

How

• New PgAgentSessionStore.pruneMessageUpdates(sessionId, messageEndSeq): walks backward from the persisted message_end via the (session_id, seq) primary-key index, bounds the contiguous message_update run immediately preceding it, and deletes only rows strictly between the run's first and last frames. O(run length), not O(session); never touches rows at/after messageEndSeq (in-flight next message).
• Runner calls it after message_end is durably appended, ordered in the write chain after that append, off the live-stream path; prune failures are logged and do not fail the run.

Safety

• No schema change; read path untouched.
• Replay output verified byte-equivalent before/after pruning.
• In-flight (not-yet-ended) frames untouched; single-frame/empty runs are no-ops.
• Crash/resume unaffected (doesn't use message_update).

Tests

• Physical first+last retention + deletion count; idempotent second call.
• Preservation of message boundaries, unrelated events, and next-message frames.
• Replay equivalence before/after pruning.
• Runner invocation with the committed message_end seq and correct ordering.
• Existing agent-session contract suite: no assertions changed, all pass.

Validation

tsc --noEmit, storage schema suites (66 passed / 37 env-gated skipped), runner suites (9 passed), ESLint, repo-wide pnpm check.