fix(testing): reconcile ORCH-116 with merged ORCH-123 (ADR renumber, CHANGELOG, env parity)

Recovery from the merge-gate rebase-conflict bounce. The feature branch was
rebased onto origin/main (which had merged ORCH-123). The single conflicting
hunk — docs/architecture/README.md — was resolved during the rebase: kept
ORCH-123's host-side staging-runner line AND the ORCH-116 test-runner bullet.

This follow-up commit reconciles the remainder:

- Renumber the global sweeping ADR adr-0049 -> adr-0050. ORCH-123 took adr-0049
  (adr-0049-host-side-docker-execution-boundary.md) on main while ORCH-116 was
  in flight, so ORCH-116 yields to the merged task and moves to the next free
  number. Mechanical cross-reference reconciliation only (git mv + title + every
  test-runner reference across README/internals/CLAUDE/CHANGELOG/config.py +
  06-adr/ADR-001 + 12-review). Main's adr-0049 host-side references are left
  byte-for-byte untouched. No design/verdict content was altered.
- Restore the ORCH-116 CHANGELOG entry that the CHANGELOG auto-merge silently
  dropped (both ORCH-123 and ORCH-116 inserted at the same [Unreleased] anchor;
  git kept only ORCH-123).
- Add the missing ORCH_TEST_RUNNER_* keys to .env.example (parity with the
  ORCH_STAGING_RUNNER_* block; ORCH-101 canon of start keys).

Refs: ORCH-116

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config.py

@@ -489,7 +489,7 @@ class Settings(BaseSettings):
     # STAGE_TRANSITIONS and the DB schema are byte-for-byte UNCHANGED — this only
     # replaces the *producer* of the artifact. Pattern = staging_runner_* / coverage_gate_*.
     # See docs/work-items/ORCH-116/06-adr/ADR-001-deterministic-test-runner.md and
-    # docs/architecture/adr/adr-0049-deterministic-test-runner.md.
+    # docs/architecture/adr/adr-0050-deterministic-test-runner.md.
     #   test_runner_enabled            -> SINGLE kill-switch (env ORCH_TEST_RUNNER_ENABLED).
     #                                     False -> the intercept never fires -> the prior
     #                                     LLM tester runs on testing via _spawn byte-for-byte