enhance(#3905): the exit-code registry — one number, one meaning, enforced at build (#3920)

* feat(#3905): the exit-code registry — one number, one meaning, enforced at build

A generated registry replaces locally-invented exit codes. Every entry records code, name, meaning, owning module and the decision that authorized it. The generator refuses to build a table where two entries claim one code, two claim one name, a code falls in a range Node or the shell reserves, 2 is claimed by anything but the hook adapter, or an allocation carries no justification. exitCodeFor is pure and total: it throws rather than returning undefined, including for prototype-chain names.

Inert by design — nothing emits a registered code until #3906. Every registered code is non-zero, asserted over the whole table, so a caller testing for failure behaves identically for pass and trips for everything else.

* feat(#3905): make the registry generator's failures machine-readable

Adds a --json mode carrying {ok, reason, context, detail}, where context is a typed payload naming the specifics the prose embedded - which code collided and under which names, which band rejected a code, which field was missing. The tests now assert on that structure instead of regex-matching the generator's stderr, which CONTRIBUTING prohibits, and the CONTEXT.md glossary gains the entry the issue's scope requires.

* test(#3905): refresh the install-tree fixtures for the new declaration

The registry declaration ships in the install tree, so all 19 golden fixtures needed regenerating. Caught by the remote matrix, not by lint:ci - the install-tree goldens are verified by a test rather than a lint, so a newly shipped file clears every local gate and fails only under the suite.

* chore(#3905): backfill changeset pr number

---------

Co-authored-by: sim <sim@local>

tests/fixtures/install-tree/opencode.json

@@ -114,6 +114,7 @@
   "gsd-core/bin/gsd_run",
   "gsd-core/bin/shared/config-defaults.manifest.json",
   "gsd-core/bin/shared/config-schema.manifest.json",
+  "gsd-core/bin/shared/exit-codes.json",
   "gsd-core/bin/shared/model-catalog.json",
   "gsd-core/bin/shared/runtime-aliases.manifest.json",
   "gsd-core/bin/verify-reapply-patches.cjs",