Fix metadata wipe on content updates + CLI metadata-search parity (v0.11.1)

[fstamatelopoulos]

Summary

Two fixes surfaced by a real incident (a content update silently cleared a document's tags — and metadata is unversioned, so the loss was unrecoverable).

1. Content updates no longer wipe metadata. Every transport defaulted an absent metadata argument to {} and the ingest RPC applied it verbatim — so any content update that didn't re-pass the tags cleared them. New contract, enforced once in cerefox_ingest_document: NULL = "not provided" → keep existing (metadata = COALESCE(p_metadata, metadata); create uses {}). Pass {} explicitly to deliberately clear. Callers fixed in lockstep: MCP handler, cerefox-ingest EF, CLI ingest/ingest-dir, frozen Python client. (The pipeline already had keep-on-absent semantics — the CLI's ?? {} default was defeating them.)

2. CLI parity: cerefox metadata search no longer hard-requires --metadata-filter — same contract as the MCP tool / EF (relaxed in v0.10.x; the CLI was missed): at least one of filter / --project-name / --updated-since / --created-since; --project-name alone lists that project's documents.

Versioning: schema 0.5.0 → 0.6.0 (RPC-only, no migration). ⚠️ Run cerefox server deploy with the release — v0.11.1 clients send NULL for absent metadata, which a 0.5.0 server would reject on update (NOT NULL column). OpenAPI → 2.1.0 (metadata description only — re-paste optional).

Backlog: the incident exposed that metadata has no recovery layer (version snapshots are content-only). Proposal with options: docs/research/metadata-versioning.md + TODO entry + plan.md note.

Test plan

• _shared: 211 pass; typecheck clean; help bundle in sync
• packages/memory: 141 ran, 0 fail (live suites gate on deployed schema ≥ 0.6.0 with a deploy hint)
• New RPC-level live test: update with p_metadata omitted → tags preserved; explicit {} → cleared (synthetic embedding, no OpenAI dependency)
• CLI update-flow asserts metadata survives four metadata-less content updates
• metadata search --project-name <p> (no filter) → lists docs; no criteria → exit 1 with guidance
• Post-merge (maintainer): cut v0.11.1 → self-update → cerefox server deploy → live suites go fully green

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