wip(929): stage 6 — drop resume_to_stage YAML lookup from projection layer

projection::project_stage was the last yaml_legacy reader in pipeline_state.
Drop the read_content+parse_front_matter detour for the "7_frozen" case and
always default resume_to to Stage::Coding. The YAML write side in apply.rs
goes in stage 7.

Story 934 (sibling refactor) will replace Stage::Frozen-with-payload with a
frozen flag orthogonal to Stage, so a story frozen in Qa stays in Stage::Qa
rather than encoding a "where to resume" target. After 934 lands the
resume_to payload disappears entirely.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/src/pipeline_state/projection.rs

@@ -131,18 +131,12 @@ pub fn project_stage(view: &PipelineItemView) -> Result<Stage, ProjectionError>
             })
         }
         "7_frozen" => {
-            // The stage to resume to is stored in front matter as `resume_to_stage`.
+            // Story 929: resume_to was previously read from YAML front matter;
-            // Fall back to Coding if the field is absent (e.g. legacy frozen items).
+            // we default to Coding here. Story 934 will obviate this — frozen
-            let resume_to = crate::db::read_content(view.story_id())
+            // becomes a flag orthogonal to Stage, so the story stays in its
-                .and_then(|content| {
+            // current Stage rather than encoding a "where to go next" payload.
-                    crate::db::yaml_legacy::parse_front_matter(&content)
-                        .ok()
-                        .and_then(|m| m.resume_to_stage)
-                        .and_then(|dir| Stage::from_dir(&dir))
-                })
-                .unwrap_or(Stage::Coding);
             Ok(Stage::Frozen {
-                resume_to: Box::new(resume_to),
+                resume_to: Box::new(Stage::Coding),
             })
         }
         other => Err(ProjectionError::UnknownStage(other.to_string())),