enhance(#2721): regenerating merge driver, regen:derived, and a name for the emitted-artifact family (#2730)

* test(#2721): failing-first suite for the gsd-regen driver and CONTEXT.md parity

Tests precede the implementation per the TDD gate. The driver module does not
exist yet, so tests/git-merge-regen-driver.test.cjs fails at require time; the
contributor-standards parity assertions fail against next as it stands today,
where the standards doc names two CONTEXT.md headings that have never existed.

Refs #2721

* feat(#2721): add the gsd-regen merge driver and regen:derived

The golden parity manifests and the two size baselines are pure functions of
the source tree, so their only correct merge is "recompute" -- something git's
ours/theirs interface cannot express. 140 of 143 conflicted-file instances
across the open PR queue are these files.

The driver deliberately does NOT regenerate. Four probes established that at
merge-driver time neither the working tree nor the index reflects the merge:
both hold the ours side, a file added by theirs does not exist yet, and
MERGE_HEAD is unwritten. Git also invokes the driver once per conflicted path
(20 here). A regenerating driver would therefore read the ours-side tree and
emit a plausible-but-wrong hash manifest -- worse than a conflict, because a
conflict is visible. So it accepts %A, runs zero subprocesses, records the
resolved paths, and prints one notice pointing at npm run regen:derived.
Staleness stays caught where it already was, by golden-install-parity in CI.

Every failure path degrades toward today's behaviour (a normal conflict).
install-tree is deliberately excluded per ADR-2719 section 7.

Also folded in, per the no-defer rule: workflow-size.cjs claimed .md files have
no eol=lf in .gitattributes; git check-attr shows eol: lf, set by .gitattributes
line 2 since #1088.

Refs #2721

* docs(#2721): document regen:derived and the gsd-regen merge driver

Adds the how-to a contributor actually reaches for when the generated parity
manifests or size baselines conflict, in both places they would look: the
merge-conflict path in CONTRIBUTING.md and the full guide in TESTING-SUITES.md,
including what the driver deliberately does not do (it does not clear GitHub's
CONFLICTING label, and it does not regenerate mid-merge).

Also scopes the new contributor-standards parity assertion to the doc's own
CONTEXT.md section. Its first run flagged `## Decision`, `## Consequences` and
`## Standards followed`, which the doc attributes to an ADR body and a PR body
rather than to CONTEXT.md -- a doc-wide extractor would have demanded CONTEXT.md
grow headings that do not belong to it.

Refs #2721

* fix(#2721): stop passing %P to the merge driver — shell injection

The isolated adversarial review found, and I independently reproduced, local
arbitrary command execution.

Git does not invoke a merge driver with an argv array. It substitutes %O %A %B
%L %P textually into the configured string and runs the whole thing through a
shell, and $(...) executes inside POSIX double quotes -- so quoting the
placeholder does not neutralise it. %O/%A/%B are git-generated temp names and
%L is an integer, but %P is the file's own path, chosen freely by any
contributor. A branch renaming a covered fixture to
evil$(touch PWNED_SENTINEL).json executed that command on the machine of every
maintainer who merged it, and the merge still reported success.

Fix removes the input rather than filtering it: %P is no longer registered, so
the driver receives no attacker-controlled argument at all. The marker records
a count instead of path names. A metacharacter filter would have been a guess
about shell grammar; passing nothing is a property. Re-ran the identical
exploit against the fixed command: nothing executed, conflict still resolved.

Two regressions guard it -- a platform-independent assertion that the
registered command carries no %P, and a real merge driven by the actual
planInstall output with a $(...) filename.

Also from review: CLI dispatch had no coverage at all (CONTRIBUTING's
"CLI and command routing" matrix), which is why runInstall/runStatus now take
{repoRoot} -- hardcoding REPO_ROOT was what made them untestable. Renamed
planResolution to resolveAndRecord since the plan* prefix promised purity it
did not have. Reconciled the eleven-vs-twelve generator count across
CONTEXT.md, CONTRIBUTING.md and the changeset.

Refs #2721

* test(#2721): scope safe.directory for the check-attr helper

The 66f4d85a run failed 11 assertions, all in the .gitattributes scoping block,
with "fatal: detected dubious ownership in repository at '/work'". The test
container checks the repo out at a path its user does not own, so git refuses
check-attr outright. Everything else passed (27,185).

`check-attr` is a pure read of .gitattributes -- no hooks, no filters -- so the
exemption is scoped to that one invocation. It is deliberately NOT applied to
the driver's own production `git config` calls, which run in the user's own
clone and should keep the protection.

Refs #2721

* test(#2721): delete the stale assertion that the driver command carries %P

The plex2 run on bdfd0856 left exactly two failures, both this test: it still
asserted the pre-fix command string, i.e. the vulnerable behaviour. Deleted
rather than relaxed, per RULESET.TESTS.delete-bad-tests -- its useful half is
already covered, in both directions, by
registeredDriverCommandNeverPassesThePlaceholderForTheFilePath.

Refs #2721

* test(#2721): drive the end-to-end merges from the real planInstall output

The e2e helper hand-rolled its own driver registration, and still carried %P.
That meant the five real-git tests were not exercising the production command
string at all -- planInstall could drift and they would keep passing. They now
register exactly what a contributor gets from npm run setup:merge-driver.

Refs #2721

* chore(#2721): backfill changeset pr number to 2730

package.json

@@ -93,6 +93,8 @@
     "gen:capability-registry": "node scripts/gen-capability-registry.cjs --write",
     "gen:registry": "node scripts/gen-registry.cjs --write",
     "gen:golden": "node scripts/gen-golden-install-parity-zcode.cjs && node scripts/gen-install-tree-fixtures.cjs",
+    "regen:derived": "npm run build && npm run gen:registry && node scripts/gen-adr-index.cjs --write && node scripts/gen-capability-matrix.cjs --write && node scripts/gen-inventory-manifest.cjs --write && node scripts/sync-manifest-versions.cjs && npm run size:baseline && npm run gen:golden",
+    "setup:merge-driver": "node scripts/git-merge-regen-driver.cjs --install",
     "validate:registry": "node scripts/validate-registry.cjs",
     "prepack": "npm run build:lib",
     "prepare": "npm run build:lib",