fix(kernel,tools,workflow): session-robustness QA sweep

Uncommitted work from the session-robustness-and-dox branch sweep
(docs/qa/QA-session-robustness-and-dox.md), verified alongside the
compression/context fixes:

- SandboxedToolExecutor: validate tool args centrally before dispatch. A
  malformed/missing-arg call becomes a recoverable ERROR: (surfaced with the
  tool's arg schema so the model can correct + retry) instead of stranding
  the stage with no artifact. + validation test.
- PlanLinter: seed artifacts (analysis) produced by the planning phase count
  as available producers, so a plan stage that `needs` them isn't flagged as
  an unproduced-need; H1 unproduced-needs + trap-state checks. + tests.
- DefaultSessionOrchestrator: live-QA robustness fixes (event-tail /
  per-stage budget + retry handling).
- workflow prompts/configs: DOX AGENTS.md alignment + freestyle/task/role
  prompt tweaks.
- SessionOrchestratorIntegrationTest: coverage for the above.
- FreestylePlanningWorkflowTest: allow list_dir in analyst tools (follows the
  list_dir wiring in 968cbfa).
- QA plan doc for the branch sweep.

examples/workflows/healthcheck.toml

@@ -15,7 +15,7 @@ id = "execute_script"
 prompt = "prompts/healthcheck_execute.md"
 needs = ["healthcheck_script"]
 produces = [{ name = "healthcheck_result", kind = "process_result" }]
-allowed_tools = ["ShellTool"]
+allowed_tools = ["shell"]
 token_budget = 2048
 
 [[transitions]]