docs(skills): spell Plugin.type as PluginType in objectstack-platform

[os-steve]

Fixes #14589

What changed

skills/objectstack-platform's copy of the Plugin interface still spelled type as
an open string with the value set carried in a trailing comment, at two sites. #13925
narrowed the published field on @objectstack/core to the closed PluginType (verified
on this branch: packages/core/src/types.ts:105 export type PluginType = 'standard' | (typeof CORE_PLUGIN_TYPES)[number];, field at :130 type?: PluginType;, and the type
is publicly re-exported via export * from './types.js'; in
packages/core/src/index.ts:21). The skill was teaching a wider accept set than the
compiler enforces.

Two same-length, no-net-growth edits per the triage scope ruling (comment
5514027764
on the card):

• skills/objectstack-platform/SKILL.md:723
• skills/objectstack-platform/rules/plugin-lifecycle.md:48

Both now read type?: PluginType; with a short pointer comment in place of the
enumeration (// closed set exported by @objectstack/core), rather than dropping the
comment outright: git grep -rn "CORE_PLUGIN_TYPES\|'theme'" etc. against
skills/objectstack-platform/ found no other site in the package naming the value set,
so per the dispatch's mechanism assumption a minimal pointer stays rather than leaving the
reader with nothing (the enumeration list itself is not restored).

⛔ No sweep of skills/** for other stale type copies, per the triage ruling's scope
limit.

Line/token budget (skills token ratchet)

Whole-file readings, before → after (origin/main 5bc2f27 → this branch):

file	lines before	lines after	tokens before	tokens after
skills/objectstack-platform/SKILL.md	1223	1223	12871	12868
skills/objectstack-platform/rules/plugin-lifecycle.md	263	263	1777	1775

Whole-package totals (skills/objectstack-platform/**, 5 files): lines 2100 → 2100,
tokens 20159 → 20154. Both files shrink individually (net deletions); the package total
shrinks; no file's token ceiling rises. node scripts/check-skills-token-ratchet.mjs
confirms: skills/objectstack-platform/SKILL.md 12868 / 12984 (-116),
skills/objectstack-platform/rules/plugin-lifecycle.md 1775 / 2408 (-633).

Reverse-check

$ git grep -n "type?: string" -- 'skills/**'
(no output)
$ git grep -n "type?: PluginType" -- 'skills/**'
skills/objectstack-platform/SKILL.md:723:  type?: PluginType;          // closed set exported by @objectstack/core
skills/objectstack-platform/rules/plugin-lifecycle.md:48:  type?: PluginType;  // closed set exported by @objectstack/core

Governed surface

skills/** is a governed surface (AGENTS.md Prime Directive #14): this PR stays draft
and is merged by hand by the maintainer after the PM's in-seat review; no agent approves
it and it does not enter the merge queue.

Gates

Derived post-final-commit with node scripts/pm/dispatch-gates.mjs --repo objectstack-ai/objectstack at 9c84135 (no paths passed): 21 command(s) apply (16 by
path match + 5 declared whole-tree). Run under scripts/pm/os-verify-lock.sh, exit codes
captured before any pipe:

• node scripts/check-ci-filter-parity.mjs — OK
• node scripts/check-closing-keyword-parity.mjs — OK
• node scripts/check-comment-mask-corpus.mjs — OK (5824 files, 0 disagree)
• node scripts/check-cross-package-test-inputs.mjs — OK
• node scripts/check-shard-attestation.mjs — OK
• node scripts/check-skills-token-ratchet.mjs — OK (see budget table above)
• node scripts/check-test-completeness.mjs — NOT MEASURED (gate's own text: "this
  gate grades a saved turbo run test log, and no log was named" — CI tees it; this card
  touches no test files)
• pnpm --filter @objectstack/lint run check:doc-formula-expressions — OK (after building
  @objectstack/formula and @objectstack/lint's dependency closure, which were not
  built in the fresh worktree)
• pnpm --filter @objectstack/spec run check:skill-docs — OK
• pnpm check:agent-test-spelling — OK
• pnpm check:corpus-claim-drift — OK
• pnpm check:cross-package-test-inputs — OK
• pnpm check:doc-authoring — OK
• pnpm check:nul-bytes — OK
• pnpm check:pm-governed-merges — OK
• pnpm check:refd-timer-probe — OK
• pnpm check:role-word — OK
• pnpm check:skill-compatibility — OK
• pnpm check:skill-frame-sync — OK
• pnpm check:skill-identifier-liveness — OK
• pnpm check:watch-hint-literal — OK

Run at 9c84135 (git rev-parse --short HEAD of this branch's tip).

This diff publishes nothing from any package (skill docs only) — skip-changeset label
applied per the os-dev clause.

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