test: make tests/ mirror the source tree; drop issue numbers from filenames

`scripts/run_tests.sh tests/<dir>/` is how a change gets its regression
coverage run, so a test filed under the wrong directory is a test nobody
runs when that code changes. Two kinds of drift had accumulated.

Parallel directories for one source package, folded into the mirror:
  tests/acp        -> tests/acp_adapter   (its __init__/conftest move with it)
  tests/cli        -> tests/hermes_cli    (prompt_toolkit fixture merged into
                                           hermes_cli/conftest.py)
  tests/run_agent  -> tests/agent         (backoff fixture becomes
                                           agent/conftest.py)
  tests/relay      -> tests/gateway/relay
  tests/state      -> tests/hermes_state

246 loose files at tests/ root, routed by the package they import/patch:
hermes_cli, hermes_state, agent, gateway, tools, plugins, tui_gateway, cron.
Installer and desktop-update script tests go to tests/scripts/{install,
desktop_update}/. 43 tests of root-level modules (batch_runner, utils,
hermes_constants, packaging) stay at the root.

Filenames drop their issue numbers (95 files: test_89315_x.py -> test_x.py);
the number stays in the module docstring where it has context.

Collisions: test_cli_skin_integration.py existed in both tests/ and tests/cli
with different subsets — merged into one (10 tests, all kept);
run_agent/test_pre_compress_memory_context.py -> agent/..._handoff.py;
tests/test_account_usage.py -> agent/test_account_usage_fetch.py;
tests/test_web_server.py -> hermes_cli/test_web_server_ws_ping.py.
Deleted: test_minisweagent_path.py (empty since PR #2804),
test_model_picker_scroll.py (tested a private copy of the logic, imported
nothing), test_process_loop_event_loop_warning.py (asserted asyncio behaviour,
imported nothing from Hermes).

Repo-root path arithmetic (Path(__file__).parents[N], dirname chains) is
bumped for the 202 files that changed depth and verified by evaluating every
such expression against the new location. classify_changes' desktop-updater
lane prefix, tests-os.yml's ignore glob and every in-tree path comment follow
the moves. tests/test_tests_tree_layout.py keeps the tree from drifting back.

AGENTS.md

@@ -330,7 +330,12 @@ scripts/run_tests.sh -v --tb=long                       # pytest flags pass thro
   `HERMES_TEST_FILE_RETRIES=0` disables). Pass-on-retry is green but printed under `⚠ FLAKY`
   with both outputs — a bug to fix, not noise. Timing tests must not assume a quiet runner:
   wall-clock bounds ≥ 2s, event-based sync, no `assert not _wait_until(...)` races.
-- **Placement:** `scripts/ci/classify_changes.py` picks jobs by changed files. A Python test
+- **Placement mirrors the source tree.** A test lives in `tests/<top-level source dir>/` (`tests/hermes_cli/`,
+  `tests/agent/`, `tests/hermes_state/`, `tests/gateway/relay/`, ...); installer/updater script tests
+  under `tests/scripts/{install,desktop_update}/`. Only tests of root-level modules (`batch_runner`,
+  `utils`, `hermes_constants`, packaging) sit directly in `tests/`. No issue numbers in filenames —
+  cite the issue in the module docstring (`test_89315_x.py` → `test_x.py`, "Regression for #89315").
+- **Placement (CI lanes):** `scripts/ci/classify_changes.py` picks jobs by changed files. A Python test
   asserting about `package.json`, `package-lock.json`, `tsconfig.json`, or `.ts/.tsx/.js/
   .mjs/.cjs` sources will not run on a JS-only PR (green on PR, red on `main` where the
   classifier fails open). Such tests belong in the vitest suite, not `tests/*.py`.