feat: support user-installed context engine plugins via $HERMES_HOME/plugins/

[whtoo]

Problem

Context engine plugins (plugins/context_engine/__init__.py) only support a single discovery path: plugins/context_engine/<name>/ inside the source tree. This is inconsistent with memory providers, which already scan both bundled (plugins/memory/<name>/) and user-installed ($HERMES_HOME/plugins/<name>/) directories.

Practical impact: user-installed context engines that live outside the hermes-agent git worktree (e.g., development symlinks) get destroyed by hermes update because git stash --include-untracked picks up untracked symlinks and git reset --hard discards them on stash-apply conflicts.

Solution

Mirror the memory provider discovery model:

• Add _get_user_plugins_dir() — profile-aware $HERMES_HOME/plugins/ resolver
• Add _is_context_engine_dir() — cheap text-scan heuristic (looks for ContextEngine or register_context_engine in __init__.py)
• Add _find_engine_dir(name) — resolves bundled first, then user path, with bundled taking precedence
• Update discover_context_engines() to scan both directories
• Update load_context_engine() to use _find_engine_dir()
• User-installed engines get isolated module namespace (_hermes_user_context_engine.*) to avoid sys.modules collisions

Behavior

• Backward-compatible: existing bundled engines load exactly as before
• Name collisions: bundled takes precedence (first-seen-wins, bundled scanned first)
• User dirs are filtered by _is_context_engine_dir() so non-engine plugins (memory providers, general plugins) in the same $HERMES_HOME/plugins/ directory are skipped
• Symlinks work: ln -s /path/to/engine ~/.hermes/profiles/main/plugins/spc_ctx survives hermes update

Files changed

File	Lines	Change
plugins/context_engine/__init__.py	+132 -44	User-level discovery support

[teknium1]

Thanks for the contribution. Current main already provides this capability through the canonical user-plugin path, so this PR is redundant.

• hermes_cli/plugins.py:1347-1352 scans $HERMES_HOME/plugins/ as the profile-scoped user plugin source.
• hermes_cli/plugins.py:614-642 provides ctx.register_context_engine() for a user plugin to register a validated ContextEngine.
• agent/agent_init.py:1755-1805 resolves context.engine through the general user-plugin system when no bundled directory engine is selected.
• tests/hermes_cli/test_plugins.py:1950-1989 covers a user plugin under $HERMES_HOME/plugins/ registering and being returned as a context engine.

This is an automated hermes-sweeper review.