fix(docs): repair dead xrefs in api.md, runbook, skill (#269)

Three internal documentation references pointed at non-existent targets:

- docs/api.md: MessageItem.content linked to #addmessage, which has no
  heading or anchor; corrected to #messageitem (the slug used by every
  other MessageItem cross-reference and matching the ### MessageItem
  heading).
- docs/cascade_runbook.md: the FD-exhaustion cross-ref used a single
  hyphen where the GitHub slug of "FD exhaustion (`os error 24` /
  EMFILE)" has a double hyphen (from the ` / ` separator); corrected to
  #fd-exhaustion-os-error-24--emfile.
- use-cases/claude-code-plugin/skills/memory-tools.md: the always-injected
  skill named two tools (search_memories, get_memory) that the MCP server
  never exposes; replaced with the real evermem_search tool and its
  params (query required, limit default 10 / max 20).

Markdown-only; no runtime behavior change.

use-cases/claude-code-plugin/skills/memory-tools.md

@@ -9,8 +9,7 @@ You have access to memory tools that can recall context from the user's past cod
 
 ## Available Tools
 
-- **search_memories**: Search past conversations using semantic + keyword matching
-- **get_memory**: Retrieve full details of a specific memory by ID
+- **evermem_search**: Search past conversations using semantic + keyword matching. Params: `query` (required), `limit` (default 10, max 20)
 
 ## When to Use Memory Search