feat(#2007): implement mempalace memory_mode kg_backend and replace routing (#2010)

Wire the two forward-declared mempalace.memory_mode modes so they actually
route recall/capture instead of silently behaving as `augment`:

- kg_backend: the palace temporal KG is the primary knowledge-graph source;
  native .planning/graphs/ is the fallback. Non-KG drawer recall stays additive.
- replace: recall resolves through the palace as the source of truth; native
  artifacts are the fallback.

Every mode stays onError:skip and default-resilient — an unreachable palace
degrades to native memory and GSD keeps writing .planning/graphs/, so no memory
is lost. Cross-mode .planning/graphs/ migration remains a documented open
question (PRD/ADR §17), out of scope here.

Surfaces updated (instruction-only contract): recall/capture commands (+ generated
skills), discuss/wave fragments, curator agent, capability.json schema. Docs:
how-to Step 3, CONFIGURATION, FEATURES, CONTEXT glossary. Regenerated
capability-registry, golden install-parity fixtures (mempalace hashes only),
agent-size-baseline. Added a routing-contract + cross-surface parity test.

Incidental (folded per no-defer rule): removed pre-existing unused imports
(spawnSync in capability-registry.test.cjs; fs in issue-498-package-identity.test.cjs)
that eslint flagged in/alongside the touched files.

Closes #2007

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.changeset/patient-voles-chatter.md

@@ -0,0 +1,5 @@
+---
+type: Changed
+pr: 2010
+---
+**MemPalace `memory_mode` `kg_backend` and `replace` are now functional** — selecting either mode now routes recall through the palace instead of silently behaving like `augment`: `kg_backend` treats the palace temporal KG as the primary knowledge-graph source (native `.planning/graphs/` as fallback), and `replace` resolves recall through the palace as the source of truth. Every mode stays default-resilient — an unreachable palace falls back to native memory and no memory is lost. (#2010)