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.

docs/api.md

@@ -793,7 +793,7 @@ attribution, so `session_id` is the only meaningful query dimension.
 | `sender_id` | `string` | Original sender id from `/add` |
 | `sender_name` | `string \| null` | Original sender name; `null` if not provided |
 | `role` | `"user" \| "assistant" \| "tool"` | Original role |
-| `content` | `string \| array<object>` | `string` for the single-text shorthand, `array` of opaque content items for the original multimodal payload (mirrors [MessageItem.content](#addmessage)) |
+| `content` | `string \| array<object>` | `string` for the single-text shorthand, `array` of opaque content items for the original multimodal payload (mirrors [MessageItem.content](#messageitem)) |
 | `timestamp` | `string` | ISO-8601 with timezone offset — see [Conventions](#conventions) |
 | `tool_calls` | `array<object> \| null` | Original tool_calls payload if any |
 | `tool_call_id` | `string \| null` | Original tool_call_id if any |

docs/cascade_runbook.md

@@ -204,7 +204,7 @@ Lives in `LanceDBSettings`; overridable via the
 `EVEROS_LANCEDB__INDEX_CACHE_SIZE_BYTES` environment variable. This
 is the only knob that bounds the steady-state file-descriptor count
 of a long-running EverOS daemon — see
-[Recovery paths § FD exhaustion](#fd-exhaustion-os-error-24-emfile)
+[Recovery paths § FD exhaustion](#fd-exhaustion-os-error-24--emfile)
 for why nothing else (prune, rebuild, `drop_index`) helps.
 
 Measured cap → FD ceiling (30 add+optimize cycles + 100-query stress