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.

examples/jsonrpc-agent/tests/fixtures/subagent/subagent-sdk/child.cordis.yml

@@ -30,6 +30,10 @@
 - id: session-checkpoints
   name: '@deepseek-ai/dsh-session-checkpoint-policy'
 
+# bash-local executes through the subprocess seam.
+- id: subprocess
+  name: '@deepseek-ai/dsh-subprocess-local'
+
 - id: bash
   name: '@deepseek-ai/dsh-bash-local'
   config: