Merge origin/master: port the SDK stack onto the subprocess seam

Master's #660 replaced dsh-subagent-subprocess with the dsh-subprocess
capability seam (ctx.subprocess + scrubbedParentEnv, tree-scoped teardown)
and moved subagent-acp onto it. Convergence for this branch's packages:

- The shared out-of-process provider vocabulary this branch had grown in
  the deleted library (NO_START_CAPABILITIES, assertPositiveFinite, cwd
  resolution, settleRunResult, subprocessRunHandle) moves into the subagent
  seam package as out-of-process.ts — it enforces subagent-seam contracts,
  not process mechanics, and both out-of-process backends now import it
  from there (subagent-acp keeps master's shape otherwise).
- subagent-sdk spawns THROUGH the SDK client (the subprocess README's
  documented exception for SDK-managed transports) and now applies the
  seam's scrubbedParentEnv() + explicit-env merge in place of the deleted
  buildChildEnv.
- sdk-client inlines the EOF→SIGTERM→SIGKILL ladder as private helpers (it
  runs outside any harness context, so it cannot ride ctx.subprocess).
- The child harness fixture gains the now-required dsh-subprocess-local
  entry for bash-local; the fixture cordis.yml keeps exercising the
  shipped provider default.

scripts/verify-package-readme-model-experience.ts

@@ -77,6 +77,8 @@ const SENTENCE_MODEL_EXPERIENCE: Readonly<Record<string, SentenceContract>> = {
   'packages/llm/token-meter': { kind: 'indirect', reason: 'The measurement service leaves model-visible changes to its consumers.' },
   'packages/lsp/lsp': { kind: 'indirect', reason: 'The provider registry delegates model rendering to dsh-tool-lsp.' },
   'packages/lsp/lsp-local': { kind: 'indirect', reason: 'The provider backend delegates model rendering to dsh-tool-lsp.' },
+  'packages/subprocess/subprocess': { kind: 'indirect', reason: 'The seam delegates all model rendering to consumer seams such as the bash executor family.' },
+  'packages/subprocess/subprocess-local': { kind: 'indirect', reason: 'The spawn backend delegates model rendering to consumer seams such as the bash executor family.' },
   'packages/sandbox/sandbox-local': { kind: 'indirect', reason: 'The provider backend delegates model rendering to dsh-bash-sandbox and dsh-tool-bash.' },
   'packages/sandbox/sandbox-policy': { kind: 'indirect', reason: 'The policy service holds the mode dsh-tool-bash and dsh-tool-fs render in their denial markers.' },
   'packages/sdk/create-sdk': { kind: 'indirect', reason: 'The initializer only writes project files; selected runtime plugins provide the generated project model surface.' },
@@ -92,7 +94,6 @@ const SENTENCE_MODEL_EXPERIENCE: Readonly<Record<string, SentenceContract>> = {
   'packages/spill/spill': { kind: 'indirect', reason: 'The storage seam delegates model rendering to spill consumers.' },
   'packages/spill/spill-local': { kind: 'indirect', reason: 'The storage backend delegates model rendering to spill consumers.' },
   'packages/subagent/subagent': { kind: 'indirect', reason: 'The provider registry delegates parent-model rendering to dsh-tool-subagent.' },
-  'packages/subagent/subagent-subprocess': { kind: 'indirect', reason: 'Only process-based subagent backends compose a child model request.' },
   'packages/support/acp-snapshot': { kind: 'none', reason: 'The test harness observes and normalizes transcripts without changing live requests.' },
   'packages/support/agent-loop-testkit': { kind: 'none', reason: 'The test helper mounts services but neither drives nor modifies model requests.' },
   'packages/support/invariants': { kind: 'none', reason: 'The observer validates requests but never rewrites their context.' },