docs(session_search): update all docs for the single-shape rewrite (#27840)

Companion PR to #27590. Sweeps remaining stale references to the
LLM-summary path that landed in main with #27590 but weren't fully
caught in the followup cleanup commit.

Real rewrites:
- user-guide/sessions.md: 'Session Search Tool' section rewritten to
  describe the three calling shapes (discovery / scroll / browse) with
  worked examples. Adds the 'Optional parameters' subsection covering
  sort and role_filter.
- user-guide/features/memory.md: 'Session Search' overview rewritten,
  comparison table updated (speed: ms instead of LLM summarization,
  added explicit free-cost row, link to sessions.md for details).

Stale-claim sweeps:
- user-guide/configuring-models.md: drop the 'Session Search' row from
  the aux-model override table (no aux model anymore), drop session
  search from the auxiliary-models list.
- user-guide/features/codex-app-server-runtime.md: drop session_search
  from the ChatGPT-subscription cost note, drop the session_search
  block from the per-task override config example.
- developer-guide/provider-runtime.md: drop 'session search
  summarization' from the auxiliary tasks list.
- developer-guide/agent-loop.md: drop session search from the
  auxiliary fallback chain list.
- user-guide/skills/.../autonomous-ai-agents-hermes-agent.md: drop
  session_search from the 'auxiliary models not working' debug step.

Untouched (still accurate as tool-name mentions, not behavioral claims):
- features/tools.md, features/honcho.md, features/acp.md
- cli.md, sessions.md (other sections)
- developer-guide/tools-runtime.md, agent-loop.md (line 157)
- acp-internals.md, adding-tools.md, prompt-assembly.md
- reference/toolsets-reference.md, reference/tools-reference.md

website/docs/developer-guide/agent-loop.md

@@ -194,7 +194,7 @@ When the primary model fails (429 rate limit, 5xx server error, 401/403 auth err
 3. On success, continue the conversation with the new provider
 4. On 401/403, attempt credential refresh before failing over
 
-The fallback system also covers auxiliary tasks independently — vision, compression, web extraction, and session search each have their own fallback chain configurable via the `auxiliary.*` config section.
+The fallback system also covers auxiliary tasks independently — vision, compression, and web extraction each have their own fallback chain configurable via the `auxiliary.*` config section.
 
 ## Compression and Persistence