ingest: dry-run / plan mode (no agent, structured output)

[aroff]

Summary

Add agwiki ingest --dry-run (or equivalent) that validates sources, resolves paths, applies UTF-8 checks, and emits a machine-readable plan (JSON or JSONL) of what would be ingested and what would be skipped (resume, caps, filters)—without invoking the agent.

Depends on

• config: evaluate cli-framework + wiki config for init-derived defaults #37 for optional defaults in output paths (nice-to-have).

Blocks

• ingest: clarify recursive batch semantics (--folder vs per-subdirectory roots) #42 (wider batch / recursive ingest should ship with dry-run first for operator safety).

Acceptance criteria

• Exit 0 on plan success; non-zero on validation errors only.
• Document interaction with --resume and future external_id (ingest: stable external id and skip-if-already-ingested (manifest or ledger) #40).