feat(runtime): gate task execution on input/output artifacts (#510)

[EnmaJim]

Linked issue

Closes #510

Summary of changes

Adds a task input/output artifact contract so a task can no longer be marked
done, nor silently feed a broken context downstream, when a required artifact
is absent, empty, or malformed.

• Shared predicate Test-DotbotArtifact (src/runtime/Scripts/Invoke-WorkflowProcess.ps1):
  exists → non-empty → (for .json) parses. Minimal, toolchain-free; Markdown/
  source bottom out at the non-empty check by design.
• Producer-exit gate strengthened: Test-TaskOutput now rejects empty/
  malformed declared outputs (previously existence-only), satisfying the issue's
  "absent, empty, or fails shape validation".
• Consumer-entry gate added: new Test-TaskInput validates a task's declared
  inputs before it runs. On failure the task is escalated to needs-input with
  a specific, actionable message (e.g. Required input 'briefing/repo-scan.md' is absent)
  via the existing Set-WorkflowTaskNeedsInput. The gate runs before any
  provider session, so a blocked task wastes no tokens.
• New first-class inputs task field threaded through the closed schemas and
  builders (TaskDefinition, TaskInstance/New-TaskInstance, New-WorkflowTask,
  _FlattenTask) plus API/MCP parity (HttpServer POST/PATCH /tasks,
  task-create MCP tool).
• Downstream blocking + resume reuse existing machinery: a needs-input task
  isn't reselected and its depends_on dependents stay blocked; answering
  "retry" requeues it and the gate re-validates — no full workflow restart.
• Tasks without inputs/outputs are unaffected (existing behaviour preserved).

Screenshots / recordings

Required input artifacts surfaced in ACTION REQUIRED, with retry/skip:

Resume: after answering "Investigate logs and retry", the task requeues with no error and runs to done:

Testing notes

• All tests green (pwsh tests/Run-Tests.ps1).
• New tests added for the input/output gate, the inputs schema, and gate wiring.

Checklist

• Tests added or updated
• Docs updated (if behaviour changed) — schema doc-comments updated; no user docs needed
• Linked issue exists
• Follows the contribution guide

[carlospedreira]

@EnmaJim this is new feature/design territory and needs its own prioritization pass, so I’ll defer it for now rather than settling the artifact contract model inside this PR.