feat(memory): optional gbrain --semantic recall for decision search

Adds gstack-decision-search --semantic (with --query): appends a 'Related from
memory' block from gbrain semantic search, scoped to the curated-memory source.
Pure enhancement, reliability-first: a new lib/gstack-decision-semantic.ts is the
ONLY decision module that touches gbrain and is imported lazily only on --semantic,
so the reliable file path never loads gbrain code. Every path degrades to the
reliable file results when gbrain is off, unconfigured, empty, or errors (never
throws, 10s timeout).

Built against the verified gbrain 0.42.x surface (text output [score] slug --
snippet, NOT JSON; curated-memory source resolved by worktree path, not a
gstack-brain-<user> id). Deterministic-contract tests only: parser units,
degrade-to-null when gbrain absent, and a fake-gbrain shim proving scope+search
end-to-end. find-contradictions deferred (no verifiable CLI surface yet + curated
memory not indexed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -913,6 +913,8 @@ enhancement layered on top, never a dependency.)
 
 - **Resurface** active decisions before re-deciding: `bin/gstack-decision-search`
   (`--recent N`, `--scope repo|branch|issue`, `--query KW`, `--all`, `--json`).
+  Add `--semantic` (with `--query`) to append related hits from gbrain memory when
+  it's up; it degrades silently to the reliable file results when gbrain is off.
   Session start already surfaces scope-relevant active decisions via Context Recovery.
   If a decision is listed, treat it as settled with its rationale; if you're about to
   reverse it, say so explicitly.