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>

docs/FEATURES.md

@@ -3149,7 +3149,7 @@ The load-bearing wire is the `plan-phase` lift: `covered` and `backstop` edges b
 - REQ-MP-02: At `plan:pre`, skill `mempalace-recall` produces `MEMORY-RECALL.md` from prior decisions, patterns, and surprises retrieved via wake-up + semantic search + KG timeline. When MemPalace is unreachable, writes an "unavailable" stub and continues.
 - REQ-MP-03: At `discuss:post`, `plan:post`, and `verify:post`, skill `mempalace-capture` files the phase artifact verbatim into the appropriate MemPalace room (`decisions`, `planning`, `milestones`). Capture is idempotent via `mempalace_check_duplicate`.
 - REQ-MP-04: At `ship:post`, agent `gsd-mempalace-curator` writes a diary entry, proposes cross-project tunnels (when `mempalace.cross_project_tunnels: true`), and runs wing-scoped sync pruning.
-- REQ-MP-05: `mempalace.memory_mode` declares three values: `augment` (default, **implemented** — palace is an additional recall layer alongside GSD native memory), `kg_backend` (**forward-declared; routing seam not yet implemented** — selecting this today behaves identically to `augment`), `replace` (**forward-declared; not yet functional** — selecting this today behaves identically to `augment`). Only `augment` has effect in the current release.
+- REQ-MP-05: `mempalace.memory_mode` has three wired values: `augment` (default — palace is an additive recall layer alongside GSD native memory, which stays authoritative), `kg_backend` (knowledge-graph queries resolve against the palace's temporal KG as the primary source, `.planning/graphs/` as fallback; non-KG drawer recall stays additive), `replace` (recall resolves through the palace as the source of truth, native memory as fallback). Every mode is `onError:skip` and default-resilient — an unreachable palace degrades to native memory and GSD keeps writing `.planning/graphs/`, so no mode loses memory. Cross-mode migration of existing `.planning/graphs/` into the palace is out of scope (not yet implemented).
 - REQ-MP-06: Every hook is `onError: skip`. No hook carries `blocking: true`. Memory never halts or fails a phase.
 - REQ-MP-07: Interactive runs prefer MCP tools; headless/cron runs prefer the MemPalace CLI (`mempalace wake-up`, `mempalace search`, `mempalace mine`, `mempalace sync`).
 - REQ-MP-08: `mempalace.auto_capture_hooks` is **forward-declared and not yet functional**. No native Claude Code hooks (`stop`, `precompact`, `session-start`) are installed by this key; the capability's hooks array is empty. This key is reserved for the future "Connected Capability" phase. Default `false`.