feat(mcp): page get_ownership_changes and summarize all changes

Between the two latest releases there are 1,568 holder changes. With only
`limit` (up to 2000, ~200 KB), agents that wanted them all overflowed
their tool output: Hermes then paged ticker by ticker (32 calls), and
opencode 1 and 2 did the same and timed out.

- `offset` pages through the rows; responses carry `offset` and
  `next_offset` (null after the last page). Pages cap at 300 rows
  (~40 KB); the over-limit error points to `offset` and `summary`.
- Rows come largest |delta| first, so the first page holds the moves
  that matter rather than tickers starting with A.
- `summary` covers every row: counts by change type, the number of
  tickers, and the ten tickers with the most changes.

The CLI's `ownership changes` is unchanged. tools.json and the website's
MCP pages are regenerated for the new description and parameter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V3syn2fN2LNo5pvTSqc6eh

crates/idx-mcp/src/params.rs

@@ -144,9 +144,12 @@ pub struct ChangesParams {
     /// Only this ticker's changes.
     #[serde(default)]
     pub ticker: Option<String>,
-    /// Maximum rows returned (default 200, at most 2000); `total` reports the full count.
+    /// Rows per page (default 200, at most 300); `total` reports the full count.
     #[serde(default)]
     pub limit: Option<usize>,
+    /// Rows to skip, for the next page: the previous response's `next_offset`.
+    #[serde(default)]
+    pub offset: Option<usize>,
 }
 
 #[derive(Debug, Deserialize, JsonSchema)]