feat(gateway): add role-scoped session search

[abhibansal-sg]

What does this PR do?

Adds an optional scope query parameter to the dashboard session-search endpoint
(GET /api/sessions/search) so a caller can restrict matches by message role. It
maps a public scope name onto the db layer's existing role_filter:

• all (default) — every role, plus direct session-id matches. Byte-identical
  to today's behaviour.
• messages — user + assistant prose only (excludes tool output).
• code — tool messages only (command output / file contents / structured results).

Any unrecognised value is treated as all. A non-all scope also drops the
session-id match pass so the result list stays a pure content-scoped result.

Related Issue

N/A

Type of Change

• ✨ New feature (additive, backward-compatible)

Changes Made

• hermes_cli/web_server.py: added a scope: str = "all" query param to
  search_sessions; normalise it to a role_filter
  (messages → ["user","assistant"], code → ["tool"], everything else →
  None); skip the session-id pass when a scope is active; pass role_filter to
  search_messages only when a scope is active.
• tests/hermes_cli/test_web_server_session_search.py: 4 new tests covering the
  default (unchanged) path, both explicit scopes, and graceful fallback for an
  invalid scope.

No db-layer change: SessionDB.search_messages already accepts role_filter
(default None = unchanged). No new env vars, hooks, or registries.

How to Test

1. scripts/run_tests.sh tests/hermes_cli/test_web_server_session_search.py
2. Manually against a running dashboard:
   • GET /api/sessions/search?q=docker and ?q=docker&scope=all → identical results.
   • ?q=docker&scope=messages → user/assistant hits only.
   • ?q=docker&scope=code → tool-message hits only.
   • ?q=docker&scope=bogus → behaves exactly like all (no error).

Checklist

• Default (scope=all / omitted) is byte-identical to current stock
• One logical, additive change; no new env vars / hooks / observers / registries
• Focused tests added (default-equivalence, explicit-scope, invalid-scope fallback)
• scripts/run_tests.sh green for the changed file + surrounding suites
• check-windows-footguns.py clean
• Conventional Commit, imperative, no trailing period