[codex] add agentic selector precondition parameter

[kssteven418]

Summary

• Add optional pre_cond support to AgenticSelectorRequest serialization.
• Expose pre_cond on both Python and Pyodide agentic_selector methods.
• Keep the field omitted from dispatch payloads when it is not provided.

Scope note

This PR was created from a clean origin/main worktree and intentionally excludes an unrelated local Pyodide workflow-trace compatibility fix that was present in the original dirty checkout.

Validation

• uv run ruff check packages/narada-core/src/narada_core/actions/models.py packages/narada-pyodide/src/narada/window.py packages/narada/src/narada/window.py
• uv run python -c 'from narada_core.actions.models import AgenticSelectorRequest; req=AgenticSelectorRequest(action={"type":"click"}, selectors={"tag_name":"BUTTON"}, fallback_operator_query="Click Submit", pre_cond="check Submit exists"); dumped=req.model_dump(); assert dumped["pre_cond"] == "check Submit exists"; req2=AgenticSelectorRequest(action={"type":"click"}, selectors={"tag_name":"BUTTON"}, fallback_operator_query="Click Submit"); assert "pre_cond" not in req2.model_dump(); print("pre_cond serialization ok")'
• git diff --check origin/main...HEAD