feat(1001): recover_half_written_items MCP tool

Adds db::recover, a discovery + recovery layer for pipeline items that
got half-written before the Stage 1 fix landed (content in content
store + SQLite shadow, no live CRDT entry). For each orphan, the
content body is re-anchored to a fresh non-tombstoned id and the old
id's content row is cleared.

Exposed as the recover_half_written_items MCP tool. dry_run defaults
to true so the caller can review what would change before mutating.

YAML front-matter parsing is hand-rolled and scoped to the three
fields the create_*_file path emits (name, type, depends_on). It
tolerates missing or malformed lines by falling back to safe
defaults; the orphan is recovered with the best metadata we can pull
from the body and the rest is left to the operator to fix up.

The discovery step is read-only and idempotent. Recovery is also
idempotent in the sense that once an orphan is lifted, the next
discovery pass won't see it.

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

server/src/http/mcp/dispatch.rs

@@ -93,6 +93,8 @@ pub async fn dispatch_tool_call(
         "purge_story" => story_tools::tool_purge_story(&args, ctx),
         // Debug CRDT dump (story 515)
         "dump_crdt" => diagnostics::tool_dump_crdt(&args),
+        // Recover half-written pipeline items (bug 1001)
+        "recover_half_written_items" => diagnostics::tool_recover_half_written_items(&args),
         // Read-only peer mesh diagnostics (story 720)
         "mesh_status" => diagnostics::tool_mesh_status(&args),
         // Arbitrary pipeline movement