wip(929): stage 10 sweep — production callsites move to CRDT, yaml_legacy shrinks

After 932 (review_hold register) and 933 (item_type + epic registers), the
remaining production yaml_legacy callers all had typed CRDT equivalents.
Migrated:

- agents/lifecycle.rs:
  - transition_to_merge_failure writes to MergeJob.error CRDT entry instead
    of YAML body. The legacy `merge_failure: "..."` front-matter write is gone.
  - reject_story_from_qa inlines the QA-rejection notes append; no longer
    needs yaml_legacy::write_rejection_notes_to_content.
  - fields_to_clear_transform helper deleted along with all five callers —
    blocked/retry_count/merge_failure are typed CRDT fields now, so clearing
    the equivalent YAML keys is redundant.

- http/workflow/pipeline.rs:
  - load_pipeline_state reads merge_failure from MergeJob.error (mirrors
    status_tools.rs).
  - validate_story_dirs checks the typed CRDT `name` register instead of
    parsing YAML front matter.

- http/mcp/status_tools.rs: review_hold reads the typed CRDT register
  (yaml_residue wrap was the last one in this file).
- http/mcp/story_tools/criteria.rs: story_name reads from CRDT.
- service/agents/mod.rs::get_work_item_content: name/agent come from CRDT.
- service/notifications/io/mod.rs::read_story_name: same.
- http/workflow/bug_ops/{bug,refactor}.rs: name-fallback paths drop YAML
  parsing in favour of the CRDT-derived item.name.

Dead helpers removed from db/yaml_legacy.rs:
  yaml_residue, write_merge_failure_in_content, write_rejection_notes_to_content,
  clear_front_matter_field_in_content, write_review_hold_in_content,
  clear_front_matter_field, write_review_hold (the last four shipped in 932).
Remaining surface: FrontMatter / StoryMetadata structs, parse_front_matter,
set_front_matter_field — kept for `coverage_baseline` writes via
test_results.rs and the generic update_story front_matter escape hatch.

Test fixtures rewritten to seed the CRDT register instead of relying on
YAML parsing during write_item_with_content:
- has_review_hold_returns_* tests
- item_type_from_id_uses_crdt_register_for_numeric_ids
- tool_list_epics_shows_member_rollup
- get_work_item_content (both copies — http/agents + service/agents)
- validate_story_dirs_missing_name_in_crdt
- server_side_merge_*_sets_merge_failure (assert MergeJob.error, not YAML)

cargo fmt --check, clippy --all-targets -- -D warnings, and the
2856-test suite all pass.

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

server/src/http/mcp/story_tools/bug.rs

@@ -5,7 +5,6 @@
 use crate::agents::{
     close_bug_to_archive, feature_branch_has_unmerged_changes, move_story_to_done,
 };
-use crate::db::yaml_legacy::parse_front_matter;
 use crate::http::context::AppContext;
 use crate::http::workflow::{
     add_criterion_to_file, check_criterion_in_file, create_bug_file, create_refactor_file,