feat(search): add --tool-output to query and tool_output to MCP search_sessions

[malihashar]

Closes #80.

Problem

txcript query and MCP's search_sessions never search tool output — command output, file contents a tool read, API responses. That content is indexed, but the default search scope (Origin::DEFAULT) excludes it, and there's no flag to opt in. A string that only appears inside a tool's output returns "no matches," even though txcript view shows it plainly.

$ txcript query ZEBRAQUUX_LINKER_FAULT
no matches for `ZEBRAQUUX_LINKER_FAULT`

$ txcript query ZEBRAQUUX_LINKER_FAULT --tool-output
claude_code  11111111-...  2026-01-02 03:04  demo
  [tool_result] error: ZEBRAQUUX_LINKER_FAULT in libfoo.a

Fix

Adds --tool-output to query and tool_output: true to search_sessions, both widening to Origin::ALL through the existing shared user_query path. The default stays unchanged (tool output is large and noisy — excluding it by default is intentional, per docs/usage.md). No changes to indexing, ranking, or output format.

Tests

New end-to-end tests drive the real CLI and MCP binaries and fail on main (unknown flag / field), passing after the fix. Library tests cover matches inside one of several tool outputs, mixed-origin sessions, Unicode/case parity with existing search, malformed tool-output shapes, and a ~96-session corpus.