bug(doctor): upgrade doctor blocked by legacy empty-content observation mutation with no local-only repair path (cloud not configured)

[eduardoalvara2]

📋 Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

📝 Bug Description

Two linked defects observed after upgrading engram 1.15.10 → 1.20.0 on a machine where cloud is not configured:

1. Capture-side: mem_session_summary (MCP, running v1.15.10 at the time) persisted a session_summary observation (#2447) with empty content, even though the MCP call included the full markdown body. mem_get_observation returned only metadata (title/session/scope/timestamps) — no content. This extends the empty-content class from fix(sync): empty prompt content accepted at capture poisons cloud push for all projects; repair cannot fix prompt entities #686 (prompt entities) to observation entities.
2. Repair-side: after upgrading to 1.20.0, engram doctor reports blocked on sync_mutation_required_fields for the queued upsert of that observation (seq=691, missing content). The suggested remediation chain dead-ends for local-only users:
   • engram cloud upgrade doctor --project <p> → upgrade_blocked_legacy_mutation_manual, "manual-action-required"
   • engram cloud repair materialize-mutations --project <p> --dry-run → tries to ping Postgres at localhost:5433 and fails, because there is no cloud store to connect to
   • Restoring the observation content locally via mem_update succeeds, but the legacy seq=691 queue row still blocks the doctor

There is no local-only path to inspect/normalize/quarantine a malformed legacy mutation when cloud was never configured — yet the mutation was enqueued anyway (see #478).

🔄 Steps to Reproduce

1. On engram 1.15.10 (Claude Code plugin MCP), call mem_session_summary with a full markdown summary → observation persists with empty content (intermittent; hit once in this session)
2. Upgrade binary to 1.20.0
3. Run engram doctor → blocked on sync_mutation_required_fields
4. Run engram cloud upgrade doctor --project "<project>" → upgrade_blocked_legacy_mutation_manual
5. Run engram cloud repair materialize-mutations --project "<project>" --dry-run → Postgres dial error (cloud not configured)
6. Repair the observation locally with mem_update (content restored, verified via search) → re-run engram doctor → still blocked (seq=691 row untouched)

✅ Expected Behavior

• mem_session_summary never persists an observation without content (validate at capture, fail loudly — same contract as fix(sync): empty prompt content accepted at capture poisons cloud push for all projects; repair cannot fix prompt entities #686 proposes for prompts)
• With no cloud target configured, either no mutation is enqueued (fix(sync): don't enqueue cloud mutations when no cloud target is configured/enrolled #478) or doctor/cloud upgrade offers a local-only remediation: inspect payload, re-materialize from the (now repaired) local observation, or quarantine the row explicitly
• Repairing the source observation locally should allow the queue entry to be re-materialized/superseded

❌ Actual Behavior

• Observation stored with empty content; cloud queue poisoned with an upsert missing required fields
• doctor permanently blocked; the only repair command requires a Postgres cloud store that does not exist on this machine
• Local mem_update fixes the data but not the queue row

💻 Environment

• OS: Windows 11 (amd64)
• Agent: Claude Code (plugin MCP engram mcp --tools=agent)
• Shell: Git Bash / PowerShell
• engram: 1.15.10 at capture time → 1.20.0 at diagnosis
• Cloud: engram cloud status → not configured

📄 Relevant Logs

[blocked] sync_mutation_required_fields — 1 finding(s) detected.
  - sync_mutation_payload_missing_required_fields: observation payload missing required fields: content
    evidence: {"entity":"observation","entity_key":"obs-c1cf8a5b698d835d","missing_fields":["content"],"op":"upsert","project":"claude migration","seq":691,"target_key":"cloud"}

$ engram cloud upgrade doctor --project "claude migration"
status: blocked
reason_code: upgrade_blocked_legacy_mutation_manual
message: manual-action-required: observation payload missing required upsert fields: content (seq=691 entity=observation op=upsert)

$ engram cloud repair materialize-mutations --project "claude migration" --dry-run
engram: cloudstore: ping postgres: failed to connect to `user=engram database=engram_cloud`:
	127.0.0.1:5433 (localhost): dial error: dial tcp 127.0.0.1:5433: connectex: No connection could be made

ℹ️ Additional Context

Related: #686 (empty prompt content poisons cloud push), #503 (single empty-content observation fails whole batch, no quarantine), #478 (mutations enqueued with no cloud target), #508 (DB maintenance toolset). The malformed seq=691 row is being kept in place on this machine as a live repro case.