fix(memory): make reconcileFacts safe under FORGE_LEDGER_ONLY (M2, partial)

[CodeWithJuber]

What & why

Partial progress on M2 (finish the ledger-only memory migration) — specifically the
one destructive step, done safely and in isolation.

reconcileFacts (src/ledger_bridge.js) tombstones any author-owned fact claim that
has no backing file in the store. Under FORGE_LEDGER_ONLY there are no fact files, so it
would tombstone every fact — silently wiping the user's memory the first time
forge recall consolidate ran. It is now a guarded no-op when ledger-only is active: the
ledger IS the store then, so there is nothing to reconcile against. A new regression test
asserts two ledger-only facts survive a reconcile (no data loss).

This is the prerequisite the rest of M2 depends on — it makes FORGE_LEDGER_ONLY=1 safe
to opt into today.

Scope note — the default flip is intentionally NOT in this PR

Flipping the FORGE_LEDGER_ONLY default to on (and deleting the legacy stores) is
deliberately deferred: piloting the flip surfaced 31 failing tests, and crucially not
all are stale default-off assertions — some expose real ledger-only path gaps that
would need fixing first, e.g.:

• brain.remember persists via a file but does not dual-write to the ledger, so under
  ledger-only the fact is lost (brain.buildIndex indexes 1 instead of 2).
• the cortex learning loop's confirm-vs-create dedup reads localLessons, whose source
  changes under ledger-only — the candidate→active lifecycle needs re-verification.

Those are correctness-critical fixes across the memory subsystem and belong in their own
reviewed change, not rushed alongside a default flip. This PR lands only the safe,
self-contained destructive-heuristic fix.

Checklist

• npm test passes (1070 pass / 0 fail / 2 gated skips — includes the new test)
• npm run check passes (Biome — 0 errors)
• New behavior has a test (no-data-loss regression)
• Conventional commit message
• CHANGELOG.md updated under ## [Unreleased]
• No new runtime dependency

Risk & rollback

• Risk level: low — narrows a destructive path to a no-op only under an off-by-default flag.
• Rollback plan: revert the single commit.

Extra checks

• npm run typecheck passes
• node src/cli.js docs check green

🤖 Generated with Claude Code

https://claude.ai/code/session_01C7htTiesXKLvAUtv6ET2jz

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Generated by Claude Code