feat(521): evict_item primitive + purge_story MCP tool

Adds the foundational capability to clear a story from the running
server's in-memory CRDT state without restarting the process. This is
story 521, motivated by the 2026-04-09 incident where stories 478 and
503 kept resurrecting from in-memory CRDT after every sqlite delete /
worktree removal / timers.json clear. The only previous remedy was a
full docker restart.

Changes:

  - server/src/crdt_state.rs: new `pub fn evict_item(story_id: &str)`.
    Looks up the item's CRDT OpId via the visible-index map, calls the
    bft-json-crdt list `delete()` primitive to construct a tombstone op,
    runs it through the existing `apply_and_persist` machinery (which
    signs, applies to the in-memory CRDT, and queues for persistence to
    crdt_ops), rebuilds the story_id → visible_index map, and drops the
    in-memory CONTENT_STORE entry. The tombstone survives a restart
    because it's persisted as a real CRDT op.

  - server/src/http/mcp/story_tools.rs: new `tool_purge_story` MCP
    handler that takes a story_id and calls evict_item. Deliberately
    minimal — does NOT touch agents, worktrees, pipeline_items shadow
    table, timers.json, or filesystem shadows. Compose with stop_agent,
    remove_worktree, etc. for a full purge. Story 514 (delete_story
    full cleanup) is the future "do it all" tool.

  - server/src/http/mcp/mod.rs: registers the `purge_story` tool in the
    tools list and dispatch table.

Usage:

    mcp__huskies__purge_story story_id="<full_story_id>"

Returns a string confirming the eviction. The story will no longer
appear in get_pipeline_status, list_agents, or any other API that
reads from the in-memory CRDT view, and on the next server restart
the persisted tombstone op will keep it from being reconstructed.

This is a prerequisite for story 514 (delete_story full cleanup) and
useful for any "kill it with fire" operator need.

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

server/src/http/mcp/story_tools.rs

@@ -67,6 +67,37 @@ pub(super) fn tool_create_story(args: &Value, ctx: &AppContext) -> Result<String
     Ok(format!("Created story: {story_id}"))
 }
 
+/// Purge a story from the in-memory CRDT by writing a tombstone op (story 521).
+///
+/// This is the eviction primitive for the four-state-machine drift problem
+/// we hit on 2026-04-09 — when a story gets stuck in the running server's
+/// in-memory CRDT and can't be cleared by sqlite deletes alone (because the
+/// in-memory state outlives any pipeline_items / crdt_ops manipulation),
+/// this tool writes a proper CRDT delete op via `crdt_state::evict_item`.
+///
+/// The tombstone op:
+///   - Marks the in-memory CRDT item as `is_deleted = true` immediately
+///     (so subsequent `read_all_items` / `read_item` calls skip it)
+///   - Is persisted to `crdt_ops` so the eviction survives a server restart
+///   - Drops the in-memory `CONTENT_STORE` entry for the story
+///
+/// This tool does NOT touch: running agents, worktrees, the `pipeline_items`
+/// shadow table, `timers.json`, or filesystem shadows. Compose with
+/// `stop_agent`, `remove_worktree`, etc. as needed for a full purge — or
+/// see story 514 (delete_story full cleanup) for a future "do it all" tool.
+pub(super) fn tool_purge_story(args: &Value, _ctx: &AppContext) -> Result<String, String> {
+    let story_id = args
+        .get("story_id")
+        .and_then(|v| v.as_str())
+        .ok_or("Missing required argument: story_id")?;
+
+    crate::crdt_state::evict_item(story_id)?;
+
+    Ok(format!(
+        "Evicted '{story_id}' from in-memory CRDT (tombstone op persisted to crdt_ops; CONTENT_STORE entry dropped)."
+    ))
+}
+
 pub(super) fn tool_validate_stories(ctx: &AppContext) -> Result<String, String> {
     let root = ctx.state.get_project_root()?;
     let results = validate_story_dirs(&root)?;