[codex] Add Maestro BPMN element plugin docs

[gozhang2]

Summary

• Adds planning.md and impl.md pairs for Maestro BPMN element plugin references.
• Covers core BPMN elements and UiPath extension-backed elements requested in the task.
• Links the new plugin references from the Maestro BPMN author capability page.

Validation

• git diff --cached --check passed before commit.
• run_workspace_checks(kinds="auto") failed on existing uv run ruff check tests findings outside this docs-only diff.
• Phase-N full gate run_workspace_checks(check_kinds=["fmt","lint","typecheck","test","test_web"]) completed: test passed (19 passed), typecheck and test_web skipped by plan, fmt/lint failed on pre-existing Python files under tests.

Notes

• Public-safe synthetic documentation only; no exported BPMN, tenant URLs, connection IDs, or temporary mission notes included.

[github-actions]

⚠️ Smoke test coverage gap — the following changed skills have no tests under tests/tasks/:

• uipath-maestro-bpmn

Consider adding smoke tests before merging.

[gozhang2]

Closed as a duplicate task PR. Rookery finalized this same tree onto the shared feature branch in PR #616 as commit 53a5b6d, so human review should happen on #616.