refactor: slim preamble resolvers + sidecar-symlink helper

Compress prose across 18 preamble resolvers — Voice, Writing Style,
AskUserQuestion Format, Completeness Principle, Confusion Protocol,
Context Health, Context Recovery, Continuous Checkpoint, Lake Intro,
Proactive Prompt, Routing Injection, Telemetry Prompt, Upgrade Check,
Vendoring Deprecation, Writing Style Migration, Brain Sync Block,
Completion Status, and Question Tuning. Same semantic contract, ~half
the bytes. Restored "Treat the skill file as executable instructions"
phrase in the plan-mode info section after diagnosing it as load-bearing.
Restored "Effort both-scales" rule in AskUserQuestion format.

Bonus: scripts/skill-check.ts gains isRepoRootSymlink() so dev installs
that mount the repo root at host/skills/gstack as a runtime sidecar
(e.g., codex's .agents/skills/gstack) get skipped instead of double-counted.

opus-4-7 model overlay gets a Fan-Out directive — explicit instruction
to launch parallel reads/checks before synthesis.

Net token impact across all generated SKILL.md files: ~140K tokens
removed across 47 outputs. Plan-* skills retain full preamble surface
(Brain Sync, Context Recovery, Routing Injection) — load-bearing
functionality that early slim attempts incorrectly cut.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

model-overlays/opus-4-7.md

@@ -1,5 +1,9 @@
 {{INHERIT:claude}}
 
+**Fan out explicitly.** When the task has independent files, claims, or review
+angles, launch parallel reads/checks before synthesizing. Keep the fan-out bounded
+and merge results before deciding.
+
 **Effort-match the step.** Simple file reads, config checks, command lookups, and
 mechanical edits don't need deep reasoning. Complete them quickly and move on. Reserve
 extended thinking for genuinely hard subproblems: architectural tradeoffs, subtle bugs,