Add sync error logging and --rebuild-cache flag

[robelkin]

Summary

• Log message IDs at WARN level when GetMessagesRawBatch returns nil (message fetch failed), in both incremental and full sync paths. Previously the error count incremented silently with no indication of which messages were affected.
• Add --rebuild-cache flag to sync and sync-full commands to optionally rebuild the Parquet analytics cache after sync. The serve command already does this automatically, but standalone CLI invocations (e.g. via launchd) left the cache stale.

Usage: msgvault sync --rebuild-cache or msgvault sync-full --rebuild-cache

🤖 Generated with Claude Code

[roborev-ci]

roborev: Combined Review (17efaac)

The PR safely introduces cache rebuilding and logging improvements, but contains a logic mismatch regarding the new rebuild flag's behavior.

Medium

• Location: cmd/msgvault/cmd/sync.go:177, cmd/msgvault/cmd/ syncfull.go:169
• Problem: The new --rebuild-cache flag does not actually force a rebuild; it only runs buildCache when cacheNeedsBuild(...).NeedsBuild is true. If a user invokes the flag to recover from a bad-but-
  not-detected cache state, the command will silently skip the rebuild even though the flag/help text says it will rebuild the analytics cache.
• Fix: Make --rebuild-cache bypass the staleness check and rebuild unconditionally, or rename/document it as a conditional refresh instead of a rebuild.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)

[roborev-ci]

roborev: Combined Review (2a9aa80)

The PR introduces a high-severity compilation error and medium-severity error handling
issues that must be addressed before merging.

High

• Location: cmd/msgvault/cmd/sync.go (around line 180), cmd/msgvault/cmd/syncfull.go (around line 172)
• Problem: The code attempts to assign
  the result of cacheNeedsBuild(dbPath, analyticsDir) to a single variable staleness and access a FullRebuild field. However, cacheNeedsBuild returns two values (bool, string), not a struct. This results in a compilation error: "assignment mismatch:
  1 variable but cacheNeedsBuild returns 2 values" and "staleness.FullRebuild undefined".
• Fix: Remove the invalid staleness check. To force an incremental cache rebuild (which exports newly synced messages), simplify the logic to directly call buildCache(dbPath, analyticsDir, false) .

Medium

• Location: cmd/msgvault/cmd/sync.go:177, cmd/msgvault/cmd/syncfull.go:169
• Problem: --rebuild-cache failures are downgraded to warnings, so the command can exit
  successfully even when the explicitly requested cache rebuild did not happen. That makes automation and user feedback inaccurate for this flag.
• Fix: Propagate buildCache errors as command failures when --rebuild-cache is set, or at minimum return a non-zero exit after sync completes.

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Synthesized from 3 reviews (agents: codex, gemini | types: default, security)