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/mod.rs

@@ -993,6 +993,20 @@ fn handle_tools_list(id: Option<Value>) -> JsonRpcResponse {
                         "required": ["story_id"]
                     }
                 },
+                {
+                    "name": "purge_story",
+                    "description": "Write a CRDT tombstone op for a story (story 521). Marks the in-memory CRDT item as deleted, persists the tombstone to crdt_ops so it survives restart, and drops the in-memory content store entry. Does NOT touch running agents, worktrees, the pipeline_items shadow table, timers.json, or filesystem shadows — compose with stop_agent / remove_worktree / etc. for a full cleanup. Use this when a story has gone zombie in the running server's in-memory state and direct sqlite deletes alone are not enough to clear it.",
+                    "inputSchema": {
+                        "type": "object",
+                        "properties": {
+                            "story_id": {
+                                "type": "string",
+                                "description": "Work item identifier (filename stem, e.g. '28_story_my_feature')"
+                            }
+                        },
+                        "required": ["story_id"]
+                    }
+                },
                 {
                     "name": "move_story",
                     "description": "Move a work item (story, bug, spike, or refactor) to an arbitrary pipeline stage. Prefer dedicated tools when available: use accept_story to mark items done, move_story_to_merge to queue for merging, or request_qa to trigger QA review. Use move_story only for arbitrary moves that lack a dedicated tool — for example, moving a story back to backlog or recovering a ghost story by moving it back to current.",
@@ -1307,6 +1321,8 @@ async fn handle_tools_call(
         "get_token_usage" => diagnostics::tool_get_token_usage(&args, ctx),
         // Delete story
         "delete_story" => story_tools::tool_delete_story(&args, ctx).await,
+        // Purge story (CRDT tombstone — story 521)
+        "purge_story" => story_tools::tool_purge_story(&args, ctx),
         // Arbitrary pipeline movement
         "move_story" => diagnostics::tool_move_story(&args, ctx),
         // Unblock story