chore(deps): scope the allowlist to a major, cover vitest.shared.mts, fix the vouch comment

All four suppressed comments from Copilot's third pass of #1962. Each was
correct.

The candidate boundary omitted `vitest.shared.mts` — root-owned, imported by
every client's vitest config, and already treated as shared non-client source
by `verify:typecheck-coverage`. It imports only Node built-ins today, which
is exactly why the omission would have gone unnoticed until a third-party
import appeared there, resolved from the root, and skewed. Shared *files* are
now a first-class list beside the shared dirs.

The allowlist was a blanket exemption by name. Each entry's rationale
establishes that a patch/minor difference is benign, which is no evidence
that a React 18-vs-19 or Hono 4-vs-5 split would be — that is a different
type surface. Being listed now tolerates skew only *within a major version*;
a cross-major skew fails like anything else. Deriving the major from the
lockfile version keeps this automatic, with no per-package range to maintain
and bump. A version that can't be parsed is treated as "cannot prove same
major" and fails rather than passes.

`verify-format-coverage.mjs`'s comment overstated the vouching graph: the
three guards do not each check the others. This one checks both siblings;
each sibling checks only this one. Described as it actually works.

The AGENTS.md rule said to bump a shared dependency in "root + all four
clients", but launcher declares no zod and `findSkew` ignores installs where
a package is absent — so the rule as written asked for spurious dependencies.
Reworded to every install that *declares* it, and mirrored in
`.github/copilot-instructions.md`.

The failure output follows: it no longer advises adding a package that is
already allowlisted, saying "MAJOR skew" instead, and it names all three
shared sources rather than the two dirs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HDdo1rNRVQnVRRdsqSrvG3

.github/copilot-instructions.md

@@ -92,7 +92,7 @@ Both exist and do different jobs. Theme files (`src/theme/<Component>.ts`) custo
 ## Gates and PR hygiene
 
 - `npm run format` before committing; **`npm run ci` before pushing** (`validate` → `coverage` → `verify:build-gate` → `smoke` → Storybook). `npm run validate` is the fast inner-loop check and is **not** a substitute — it runs `test`, not `test:coverage`, so it does zero coverage gating.
-- **A dependency bump must land in every install.** v2 is not a workspace — the root and each `clients/*` have their own `node_modules`, and a client's test project compiles `core/` and `test-servers/src` (which resolve from the **root**) alongside the client's own sources. Bumping a shared dependency in one manifest only puts two versions of it in one `tsc` program; for a recursive-generic surface like zod that exhausts the tsc heap (#1896). `verify:dep-lockstep` fails the build on this, so a PR touching one `package.json`/`package-lock.json` for a package `core/` or `test-servers/src` imports should touch all of them.
+- **A dependency bump must land in every install that declares it.** v2 is not a workspace — the root and each `clients/*` have their own `node_modules`, and a client's test project compiles `core/` and `test-servers/src` (which resolve from the **root**) alongside the client's own sources. Bumping a shared dependency in one manifest only puts two versions of it in one `tsc` program; for a recursive-generic surface like zod that exhausts the tsc heap (#1896). `verify:dep-lockstep` fails the build on this, so a PR bumping a package that the shared sources import should update the root **and every client that already lists it** — not every client unconditionally, since a package absent from an install can't skew and adding it there would be a spurious dependency.
 - **Every PR references an issue**, first body line `Closes #<ISSUE_NUMBER>`.
 - **Every PR carries exactly one version label**, `v1` or `v2`, matching its base branch.
 - Update the relevant `README.md` / `AGENTS.md` when a change adds, removes, renames, or repurposes a file or folder, changes the structure or tech stack, or introduces a command, dependency, or architectural pattern.