feat(context-engine): host contract for external context engines

Condenses the substance of PRs #16453, #17453, #16451, #17600, and #13373
into a minimal generic host contract that external context engine plugins
(e.g. hermes-lcm) need to integrate cleanly. Drops scaffolding that
duplicated existing infrastructure or had marginal value.

Five concrete changes:

1. `_transition_context_engine_session()` on AIAgent — generic lifecycle
   helper that fires on_session_end → on_session_reset → on_session_start
   → optional carry_over_new_session_context. Engines implement only the
   hooks they need; missing hooks are skipped. Built-in compressor keeps
   its existing reset-only behavior because callers default to no
   metadata. `reset_session_state()` now optionally accepts
   previous_messages / old_session_id / carry_over_context and delegates
   to the transition helper when provided. (#16453)

2. `conversation_id` passed to `on_session_start()` — both the
   agent-init call site and the compression-boundary call site now
   forward `self._gateway_session_key` so plugin engines have a stable
   conversation identity that survives session_id rotation (compression
   splits, /new, resume). The key already existed on AIAgent; it just
   wasn't reaching engines. (#16453)

3. Canonical cache buckets forwarded to engines — the usage dict passed
   to `update_from_response()` now includes input_tokens, output_tokens,
   cache_read_tokens, cache_write_tokens, and reasoning_tokens on top of
   the legacy prompt/completion/total keys. Engines can make decisions on
   cache-hit ratios and reasoning costs instead of only aggregates. ABC
   docstring updated. (#17453)

4. Plugin-registered context engines visible in the picker —
   `_discover_context_engines()` in plugins_cmd.py now also includes
   engines registered via `ctx.register_context_engine()` from plugin
   manifests, deduplicating by name so repo-shipped descriptions win on
   collision. (#16451)

5. `_EngineCollector.register_command()` — context engines using the
   standard `register(ctx)` pattern can now expose slash commands (e.g.
   `/lcm`). Routes to the global plugin command registry with the same
   conflict-rejection policy regular plugins use (no shadowing built-ins,
   no clobbering other plugins). Previously these calls hit a no-op and
   the slash commands silently never appeared. (#17600)

Dropped from the original 5 PRs:

- Compression boundary signal (`boundary_reason="compression"`) from
  #16453 — already on main at `agent/conversation_compression.py:412-424`,
  landed via the bg-review extraction.

- `discover_plugins()` before fallback in run_agent.py from #16451 —
  redundant: `get_plugin_context_engine()` already routes through
  `_ensure_plugins_discovered()` which is idempotent.

- Runtime identity diagnostics method + helpers from #13373 (+251 LOC) —
  operators can already read engine state via `engine.get_status()`;
  the diagnostics view added marginal value relative to its surface area.

- The 553-LOC slash-command machinery from #17600 — replaced with a
  20-LOC `register_command` method on the collector that reuses the
  existing plugin command registry instead of building a parallel one.

Net: ~215 LOC of host-contract changes + 282 LOC of focused tests, vs
~1,176 LOC across the original 5 PRs.

Co-authored-by: Tosko4 <1294707+Tosko4@users.noreply.github.com>

Closes #16453.
Closes #17453.
Closes #16451.
Closes #17600.
Closes #13373.
Related: stephenschoettler/hermes-lcm#68.

agent/conversation_loop.py

@@ -1769,10 +1769,19 @@ def run_conversation(
                     prompt_tokens = canonical_usage.prompt_tokens
                     completion_tokens = canonical_usage.output_tokens
                     total_tokens = canonical_usage.total_tokens
+                    # Forward canonical token + cache buckets so context engines
+                    # can make decisions on cache hit ratios / reasoning costs,
+                    # not just legacy aggregate tokens. Legacy keys stay for
+                    # back-compat with engines that only read prompt/completion/total.
                     usage_dict = {
                         "prompt_tokens": prompt_tokens,
                         "completion_tokens": completion_tokens,
                         "total_tokens": total_tokens,
+                        "input_tokens": canonical_usage.input_tokens,
+                        "output_tokens": canonical_usage.output_tokens,
+                        "cache_read_tokens": canonical_usage.cache_read_tokens,
+                        "cache_write_tokens": canonical_usage.cache_write_tokens,
+                        "reasoning_tokens": canonical_usage.reasoning_tokens,
                     }
                     agent.context_compressor.update_from_response(usage_dict)