docs: add skip column config option for conditional column generation (#479) (#480)

* plan: add skip_when for conditional column generation (#479)

Adds implementation plan for a `skip_when` field on `SingleColumnConfig`
that enables conditional column generation. When the Jinja2 expression
evaluates truthy, the cell is set to None and the generator is skipped.
Skips auto-propagate through the DAG to downstream columns.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* plan: remove HopChain example from skip_when plan

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* plan: replace HopChain example with generic product review example

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* plan: add open questions on skip sentinel value and row filtering

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* plan: major revision — SkipConfig model, sync engine support, decouple propagation

- Introduce SkipConfig(when, value) as nested model on SingleColumnConfig
- Move propagate_skip to SingleColumnConfig as independent field, fixing
  bug where columns with no SkipConfig couldn't participate in propagation
- Add full sync engine implementation (Steps 4a-4d) covering both
  _fan_out_with_threads and _run_full_column_generator dispatch paths
- Add serialization boundary stripping for both DatasetBatchManager (sync)
  and RowGroupBufferManager (async)
- Simplify architecture diagrams for readability
- Update all references, design decisions, verification plan

Made-with: Cursor

* updates

* plan: document get_required_columns for skip propagation

- Explain why propagation must not use get_upstream_columns() once
  skip.when adds DAG edges; add _required_columns and
  get_required_columns() to the execution graph plan
- Point async _run_cell at get_required_columns for parity with sync
- Clarify DropSkippedRowsProcessorConfig vs stripping __skipped__ for
  DataFrames; tighten resolved-questions wording
- Extend DAG/graph verification with gating_col regression case

Refs #479

Made-with: Cursor

* plan: centralize __skipped__ handling in skip_provenance

- Document new skip_provenance.py (key constant, read/write/strip API)
- Point sync builder, async scheduler, and batch buffers at shared helpers
- Strip metadata before every DataFrame from buffer dicts, including
  FULL_COLUMN active subsets
- Split §3 into skip_evaluator vs skip_provenance; extend verification

Refs #479

Made-with: Cursor

* plan: align doc title with SkipConfig / skip.when

Drop legacy skip_when naming in headings and #362 cross-reference.

Refs #479

Made-with: Cursor

* plan: address review — delimiter validation, centralized error handling, caller-owns-deserialization

- SkipConfig._validate_when_syntax now checks find_undeclared_variables
  is non-empty, rejecting expressions without {{ }} delimiters that
  would silently skip every row
- evaluate_skip_when centralizes try/except so both sync and async
  engines get identical fail-safe behavior on eval errors
- evaluate_skip_when takes a single pre-deserialized record; caller
  runs deserialize_json_values once and passes to both skip eval and
  generator (no double deserialization, no redundant parameter)
- Update _should_skip_cell, async _run_cell, Files Modified table,
  and verification section accordingly

Refs #479

Made-with: Cursor

* plan: add get_side_effect_columns accessor to execution graph spec

Document _side_effects_by_producer inverse map and
get_side_effect_columns() accessor on ExecutionGraph, needed by
_write_skip_to_record / apply_skip_to_record to clear __trace,
__reasoning_content, etc. on skip. Added to both Step 2b metadata
section and Files Modified table.

The __skipped__ leak into active_df (greptile's other P1) was already
fixed in 7046378 via strip_skip_metadata_from_records.

Refs #479

Made-with: Cursor

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nabinchha