S0090-cli-exit-2: the exit-code contract cites sysexits accurately

[NSchatz]

Spec: work/specs/S0090-cli-exit-2/spec.md (umbrella).

The defect

The CLI publishes an exit-code contract and grounds it in the Unix sysexits.h header. Under that
framing it labelled exit 2 as EX_USAGE. sysexits.h assigns EX_USAGE the value 64. A
developer aligning their own tooling to the header therefore learned a false fact about it from our
documentation, on the surface an editor renders on hover and the surface a docs site serves.

The label sat in two places in src/core/exit-codes.ts (the module contract table and the USAGE
member JSDoc, which compiles into dist/*.d.ts), and README.md plus
docs-content/concepts-archetype.md grounded their whole table in that header while listing 1 and
2, which it defines at neither value.

What changed

• Exit 2 carries no sysexits constant. 1 and 2 are both stated as this CLI's own values, in
  the module contract, in each of their table rows, and on both published pages.
• The four labels that were already correct (EX_DATAERR 65, EX_NOINPUT 66, EX_UNAVAILABLE 69,
  EX_SOFTWARE 70) are untouched.
• test/exit-code-docs.test.ts is new. It sweeps the README, every source file and every published
  docs page, binds each sysexits constant to the number that surface attributes it to through three
  association rules (member doc comment, table row, nearest number on the line), and reds when the
  two disagree, naming the surface, the line and the code.

What did not change

• No published number moved. 0/1/2/65/66/69/70 are where they were.
• No member of the exported EXIT map was added, removed or renamed.
• test/exit-code-matrix.test.ts is byte identical to its content on the base commit:
  git diff --exit-code origin/main...HEAD -- test/exit-code-matrix.test.ts is empty.
• No runtime behaviour. No dependency. No docs-content/sidebars.json or docs IA change.

The new guard is not vacuous

Two vacuity traps have sprung in this suite before, so the guard asserts the premise on each side:
the sweep's population is asserted positive and the specific files named; both association rules are
asserted to bind the four correct labels to 65/66/69/70 against the real map; the exact text that
shipped before this correction is replayed as a negative control and must be reported; and that
control has its own control, three correct shapes that must stay silent over a population asserted
non-empty.

Checked end to end as well: restoring the three surfaces from origin/main with this test in place
turns 7 of its 22 cases red, including

src/core/exit-codes.ts:13: EX_USAGE is attributed to exit 2, but sysexits.h assigns it 64
src/core/exit-codes.ts:45: EX_USAGE is attributed to exit 2, but sysexits.h assigns it 64

which are the two defect sites this change removes.

Gates

pnpm typecheck, pnpm lint, pnpm format:check, pnpm check:no-emdash (11 binary excluded, the
count CLAUDE.md records), pnpm check:no-internal-refs, pnpm check:agent-notes, pnpm test
(38 files, 690 tests) and pnpm test:coverage (core 97.84 / 93.95 / 93.28 / 98.12, commands
98.53 / 94.18 / 100 / 99.68, both above the per-directory 90 gate) all pass.

Changeset (patch) and a [Unreleased] changelog entry are included.