fix(stats): M6 — trekbrief and trekplan records land where the yardstick reads

New lib/stats/stats-append.mjs: one record on stdin, appended to
<data dir>/<kind>-stats.jsonl. The data dir is CLAUDE_PLUGIN_DATA, else
~/.claude/plugins/data/voyage-ktg-plugin-marketplace (the yardstick's
default). A failed write exits 1 with a reason: reported, never silent.
/trekbrief Phase 7 and /trekplan Phase 12 now run it instead of
"append to ${CLAUDE_PLUGIN_DATA}/…; skip silently". intent-approval's
resolveApprovalDataDir delegates to the same resolver.

Chose a stdin heredoc over a --json argument because a JSON record in a
shell argument breaks on quotes in task text. Only trekbrief and trekplan
are rewired (økt 2's countable form needs exactly those two); the other
commands' stats prose is unchanged and docs/architecture.md says so.

Red a358059 7/7 → green 7/7. Suite 1208: 1206 pass / 0 fail / 2 skip.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture.md

@@ -129,6 +129,12 @@ Stats:
 - Research stats: `${CLAUDE_PLUGIN_DATA}/trekresearch-stats.jsonl`
 - Continue stats: `${CLAUDE_PLUGIN_DATA}/trekcontinue-stats.jsonl`
 
+The Bash tool env carries no `CLAUDE_PLUGIN_DATA`. Brief and plan stats are
+written by `lib/stats/stats-append.mjs`, which falls back to
+`~/.claude/plugins/data/voyage-ktg-plugin-marketplace/`, the directory
+`scripts/yardstick.mjs` reads. A failed write is reported, not skipped.
+The other commands' stats phases still name the variable in prose only.
+
 ## Terminology
 
 - **Task brief** — produced by `/trekbrief`. Declares intent, goal, and research plan. Drives planning.