deep-search: add stable API command with read/list flows

Implement a new top-level src deep-search command using the stable Sourcegraph 7.0 endpoints under /api/deepsearch.v1.Service/*.

This command now supports:
- create + async wait flow (CreateConversation + GetConversation polling)
- read existing conversations via -read (resource name, numeric id, URL)
- list conversation summaries via -list and -limit
- JSON output mode (-json) and plain-text answer output

Migration compatibility details:
- sends stable CreateConversation payload shape with nested conversation/question input blocks
- reads stable top-level conversation state oneof (processing/completed/error/canceled)
- parses stable answer blocks (answer[].markdown.text)
- keeps a legacy fallback only for read-token/shared URL lookups, since token lookup is not exposed in the stable API

Also updates top-level command help text and adds focused tests for polling, payload parsing, identifier parsing, read-token fallback, and summary extraction.

Author: keegancsmith