fix(gen): preamble tiers are explicit; silent ?? 4 default becomes an error; spec stops rendering its preamble twice

Eight skills (scrape, diagram, spec, skillify, pair-agent, landing-report,
open-gstack-browser + its connect-chrome symlink) silently received the
HEAVIEST tier-4 preamble because a missing frontmatter field defaulted to 4.
Tiers are now declared in every {{PREAMBLE}} template's frontmatter and a
missing declaration throws at generation time with the template path (the 5
templates without {{PREAMBLE}} never invoke the resolver). The stale
hand-written tier-map comment (wrong in 3 of 4 rows) is gone.

Bonus bug fixed: spec/SKILL.md.tmpl mentioned {{PREAMBLE}} in prose, so the
generator inlined the ENTIRE preamble a second time — spec/SKILL.md shrinks
127,462 -> 80,924 bytes (-46,538) from de-duplication alone. skill-size-budget
gains a reasoned INTENTIONAL_SHRINKS entry (its frozen baseline had measured
the doubled-preamble bug). New tests: missing-tier throw carries the path;
every {{PREAMBLE}} template declares a tier. (Carries chunk-23 edits in the
shared test/gen-skill-docs.test.ts.)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

spec/SKILL.md.tmpl

@@ -1,5 +1,6 @@
 ---
 name: spec
+preamble-tier: 4
 version: 0.1.0
 description: |
   Turn vague intent into a precise, executable spec in five phases. Files the issue,
@@ -286,8 +287,8 @@ contributor's "match template to content" rules.
 
 #### Phase 5 dispatch logic (plan-mode-aware default)
 
-Read `GSTACK_PLAN_MODE` from the environment (emitted by `{{PREAMBLE}}`'s
-preamble bash). Then:
+Read `GSTACK_PLAN_MODE` from the environment (emitted by the preamble bash at
+the top of this skill). Then:
 
 1. **`--file-only` or `--no-execute` flag present** → file-only path.
 2. **`--execute` flag present** → file + spawn path.