feat(seed): support explicit checkpoint resume

[RerankerGuo]

Background

The seed command already persists per-session capture and L1 cursors in
.metadata/checkpoint.json, but it unconditionally rejects every output
directory containing a checkpoint. An interrupted historical import therefore
cannot reuse its durable progress and must start from an empty directory.

This PR adds a conservative, explicit resume mode. It replays the full input
through the existing cursor-based capture path, which skips already captured
timestamps and continues with appended rounds.

Refs #115.

Changes

• Add --resume to openclaw memory-tdai seed.
• Require all resume preconditions before starting:
  • an explicit --output-dir;
  • an existing output directory;
  • an existing .metadata/checkpoint.json;
  • stable timestamps already present on every input message.
• Preserve the existing fail-safe behavior without --resume: checkpointed and
  non-empty directories remain rejected.
• Move output-plan validation before timestamp confirmation so an invalid resume
  fails without prompting or mutating normalized input.
• Add six focused tests for valid resume and every rejection branch.
• Document the append-only resume contract and example command.

Why stable timestamps are mandatory

Auto-filled timestamps use the current time. Generating them again on a resumed
run would make old messages appear new and duplicate the import. Requiring
source timestamps makes the existing per-session cursor idempotent without a
new checkpoint schema or input-side index.

Out of scope

• No checkpoint format or storage migration.
• No arbitrary mid-file rewrite detection.
• No change to L1/L2/L3 scheduling.
• No automatic resume; users must opt in with --resume.

Verification

• npm test -- src/cli/commands/seed.test.ts — 6 passed
• npm test — 73 passed
• npm run build — passed
• npm pack --dry-run — passed, package size 716.4 kB
• git diff --check — clean

Impact

• Breaking change: No.
• Default behavior: Unchanged.
• Persistence: Reuses existing checkpoint fields; no schema changes.
• Safety: Resume is rejected unless the input can be replayed
  deterministically.

Checklist

• Resume requires explicit user intent.
• Missing/unstable inputs fail before pipeline startup.
• Existing overwrite protections remain intact.
• Focused and full test suites pass.
• Full build and package dry-run pass.
• Documentation includes constraints and example usage.

[Maxwell-Code07]

Thank you for your interest and contributions! We will review your code and get back to you as soon as possible!