fix(memory): complete discovery and registration parity for out-of-tree providers

Builds on the three salvaged commits: adds the sources and integration points
they leave out, so a pip-installed memory provider is not a second-class
citizen next to a directory install.

Discovery
- Project-local providers (./.hermes/plugins/<name>/), gated on
  HERMES_ENABLE_PROJECT_PLUGINS exactly as PluginManager gates its own project
  scan. Completes the four sources CONTRIBUTING.md and AGENTS.md already
  promised; memory was the only discovery system missing two of them.
- find_provider_dir() now resolves a package entry point to its directory.
  This is load-bearing: config_schema.py (the dashboard panel) and cli.py (the
  `hermes <provider>` subcommands) are read from disk rather than imported, so
  without a directory a pip-installed provider silently lost both.
- list_memory_provider_names() includes entry-point providers, so they appear
  in the dashboard's memory.provider dropdown.

Resolution stays import-free. hermes_cli.plugins.resolve_module_origin() is
extracted from _resolve_module_source() (added by the salvaged #76567) and
shared, so discovery walks a module's file layout instead of importing it.
find_provider_dir() is called from the dashboard and from argparse setup, long
before the operator has chosen a provider — importing every installed candidate
would execute third-party code on the strength of a package being present.
A test asserts the resolution leaves no side effects and no sys.modules entry.

Registration
- PluginContext gains register_memory_provider(). Memory was the only provider
  category without one; context engine, image gen, video gen, web search,
  browser, TTS, transcription, secret source, dashboard auth and platform all
  have one.
- _ProviderCollector delegates unknown register_* calls to a real
  PluginContext instead of carrying three hand-written no-ops. It silently
  dropped register_tool/register_hook, and had no register_auxiliary_task at
  all — despite PluginContext.register_auxiliary_task documenting a memory
  provider (hindsight's pre-retain dedup) as its worked example. It can no
  longer drift behind PluginContext.
- A raise after register_memory_provider() no longer costs the provider. The
  loader caught it into a debug log, discarded the registered instance, and
  fell through to "instantiate any MemoryProvider subclass" — returning a
  different, unconfigured provider. A silent downgrade that looked like
  success, and the exact outcome of calling register_auxiliary_task.

Activation is unchanged: still gated on memory.provider naming the plugin, and
covered by a test so the real PluginContext cannot start requiring
plugins.enabled — that would break every existing user-installed provider.

Verified end to end against a real third-party provider (kainappsinc/elephant)
installed by pip alone, with no directory copy: it appears in the dropdown,
resolves its directory, loads with its tools, and renders its dashboard panel.

Closes #40101.

AGENTS.md

@@ -828,6 +828,14 @@ Separate discovery system for pluggable memory backends. Current built-in
 providers include **honcho, mem0, supermemory, byterover, hindsight,
 holographic, openviking, retaindb**.
 
+Discovery covers the same four sources as the general `PluginManager` —
+bundled, `$HERMES_HOME/plugins/`, `./.hermes/plugins/` (opt-in via
+`HERMES_ENABLE_PROJECT_PLUGINS`), and `hermes_agent.memory_providers` entry
+points — but with **bundled-first** precedence, the reverse of the general
+system's later-wins order: a memory provider is activated by name, so a
+dropped-in directory must not be able to shadow a shipped one. Discovery
+enumerates without importing; nothing runs until `memory.provider` names it.
+
 Each provider implements the `MemoryProvider` ABC (see `agent/memory_provider.py`)
 and is orchestrated by `agent/memory_manager.py`. Lifecycle hooks include
 `sync_turn(turn_messages)`, `prefetch(query)`, `shutdown()`, and optional