fix(updex): refuse legacy sysext runners instead of falling back to SysextDir

[bketelsen]

Summary

ClientConfig.SysextRunner was the one hole in the ADR-0011
capture-at-construction invariant. A runner implementing only the original
four-method sysext.SysextRunner was linked through its pathless
LinkToSysext, and Client.sysextLinkDirForRunner() returned the mutable
package-global sysext.SysextDir — read at call time, long after NewClient.
A client configured with RuntimePaths.SysextLinkDir therefore linked
(and snapshotted its catalog rollback state) somewhere else entirely, while
docs/specs/sdk-api.md asserted that "mutating ... sysext.SysextDir cannot
redirect the client".

This change removes the fallback rather than papering over it:

• Client.linkToSysext links only through sysext.PathSysextRunner, with
  c.paths.sysextLinkDir. sysextLinkDirForRunner is gone; the catalog
  rollback snapshot (updex/catalog.go) uses the captured directory directly.
• New exported sentinel updex.ErrLegacySysextRunner (testable with
  errors.Is). Every non-dry-run operation that may end in a link checks
  requireLinkableRunner() before it mutates anything: CatalogAdd
  before writing a definition, EnableFeature with Now before writing the
  drop-in, installTransfer before removing a legacy CurrentSymlink or
  downloading. A legacy runner therefore fails cleanly instead of part-way
  through an install into the wrong directory.
• Dry runs never link, so they remain available to any runner.
• The SysextRunner interface is untouched: existing implementations still
  compile, and adding LinkToSysextAt is what makes one usable for a real
  install.
• sysext.MockRunner implements PathSysextRunner (compile-time assertion
  included) and records the directory it was handed in LinkToSysextAtDir;
  LinkToSysextCalled is still set by either entry point, so existing
  assertions are unaffected.
• docs/specs/sdk-api.md and docs/design/overview.md now state the legacy
  behavior precisely instead of overstating client isolation.

Resolves the Snowcat architecture-gap item
28c975eb-f265-4597-aefb-d4c66080df4a.

The only remaining sysext.SysextDir read in non-test updex code is
resolveRuntimePaths (updex/updex.go:142) — the construction-time capture
itself.

Checks

• make fmt — code is formatted (gofmt -l clean; make ci's gofmt
  stage passes)
• make ci — tidy, vet, gofmt, lint (.golangci.yml), unit tests, the
  80.0% coverage floor (make test-coverage-check then
  make coverage-check), race tests, linux amd64/arm64 builds

==> verify: go.mod is tidy
==> verify: go vet
==> verify: gofmt
==> lint (golangci-lint 2.13.1)
==> unit tests with coverage
==> coverage floor
==> end-to-end tests
==> race detector
==> cross-architecture build
==> CI gate passed

• CLI/e2e changes: covered by make ci's ./tests/e2e/... stage (green);
  no CLI surface changed by this PR
• New or changed behavior has focused tests, including failure paths

New tests in updex/install_link_test.go, all against a client constructed
with an instance SysextLinkDir whose package global is redirected to a
different temp dir after construction:

• TestUpdateFeatures_LegacyRunnerRefusedBeforeAnyMutation — the legacy
  runner's LinkToSysext is never called, the redirected global dir and the
  captured link dir are both empty, and the target dir still holds only the
  staged image.
• TestEnableFeature_Now_LegacyRunnerRefusedBeforeDropIn —
  errors.Is(err, ErrLegacySysextRunner) and no drop-in was written.
• TestEnableFeature_DryRunNowStillWorksWithLegacyRunner — the dry-run path
  is unaffected.
• TestUpdateFeatures_PathRunnerReceivesCapturedLinkDir — a
  PathSysextRunner is handed the captured dir, never the moved global.

Falsification: with the guards removed and the old fallback restored, the two
refusal tests fail (expected UpdateFeatures to refuse a legacy sysext runner, EnableFeature error = <nil>, want ErrLegacySysextRunner); with the
fix in place all seven link tests pass. The pre-existing
DefaultRunner-backed link tests (RestoresSysextLinkForCurrentImage,
LeavesCorrectSysextLinkAlone, EnableFeature_Now_RestoresSysextLinkForCurrentImage)
are unchanged and still green.

Risk classification

• Tier 1: Low
• Tier 2: Moderate
• Tier 3: High

Rationale:

• The diff touches sysext/**, a review-required protected boundary in
  policies/agent-governance.json (installation-and-update) with a
  minimum_risk_tier of high, and it changes a privileged filesystem-write
  path: which directory a client is allowed to create sysext symlinks in.
  It is also a behavior break for any out-of-tree SysextRunner that lacks
  LinkToSysextAt — such a runner now returns ErrLegacySysextRunner
  instead of silently linking into /var/lib/extensions. That refusal is the
  point (the old behavior wrote outside the client's declared directory), but
  it is a compatibility change and takes the highest applicable tier.

Docs housekeeping

• README.md, docs/design/overview.md, docs/specs/* updated for
  behavior changes; AGENTS.md for convention/workflow changes —
  docs/specs/sdk-api.md (the NewClient isolation paragraph and the
  sysext package reference) and docs/design/overview.md (testing
  patterns) updated; no README.md or AGENTS.md change is implied.
• New docs started from their category's TEMPLATE.md and indexed in
  docs/README.md — no new docs; existing docs edited in place.
• New significant decision recorded as an ADR first, in this PR — not
  applicable: this enforces the existing ADR-0011 invariant rather than
  deciding anything new.
• Conformance aliases (ADR-0012) untouched — canonical targets edited
  instead.

Verification

• node scripts/check-docs.mjs green

ok   docs_index_coverage: 1.000 (required 1)
ok   link_integrity: 1.000 (required 1)
ok   symlink_resolution: 1.000 (required 1)
checked: 20 docs, 277 links, 9 symlinks

make test-docs-check also green (test-check-docs: all assertions passed).

• Checked against the
  PR review rubric

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 72.41379% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
sysext/mock_runner.go	0.00%	4 Missing ⚠️
updex/catalog.go	50.00%	1 Missing and 1 partial ⚠️
updex/install.go	87.50%	1 Missing and 1 partial ⚠️

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