fix(#1164): wire external-job config keys + document wave:post choice (#2006)

Refinements A-D to the external-job capability (PR #1998 follow-up):

A. Document why the contribution registers at execute:wave:post: #1164 asks
   for wave:pre, but execute-phase.md only dispatches wave:post today (wave:pre
   is declared in the loop host contract but not rendered). Wiring wave:pre is
   a core-loop change #1164 puts out of scope; the executor honors the
   runtime_budget classification guidance before running any tagged task.

B. external_job.artifact_dir is now consumed (was declared but unused): the
   adapter resolves it via the canonical capability-config seam and surfaces
   the resolved root in submit output.

C. external_job.submit_timeout_ms / poll_timeout_ms are now read from config
   (were shadowed by env-only reads). Precedence: env > config > registry
   default; non-numeric config values fall back (no guessing, no NaN).

D. CLI surface gains unit coverage: parseFlags, findPlanningDir,
   resolveExternalJobSettings, formatShowReport.

Regenerates capability-registry.cjs from the updated capability.json.

.changeset/external-job-config-wiring.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 2006
+---
+**Setting `external_job.submit_timeout_ms` / `poll_timeout_ms` / `artifact_dir` in `.planning/config.json` now actually configures the SLURM adapter** — the keys were declared by the external-job capability but the adapter only read env vars, so config edits silently had no effect. The adapter now resolves them through the canonical capability-config seam (env override > config > registry default), surfaces the resolved `artifact_dir` in `submit` output, documents why the contribution registers at `execute:wave:post` (#1164 asks for `wave:pre`, which `execute-phase.md` does not dispatch today; wiring it is a core-loop change #1164 explicitly defers), and gains unit coverage for the CLI surface (`parseFlags`, `findPlanningDir`, `resolveExternalJobSettings`, `formatShowReport`). (#1164)