parse_frontmatter's malformed-YAML fallback stores every value as a string,
so a skill's `metadata` can be a str. `_category`/`_related` chained
`.get("metadata", {}).get("hermes", {})` and blew up with `'str' object has
no attribute 'get'`, taking down `build_learning_graph()` (and thus /journey
and `hermes journey`) whenever any installed skill had bad frontmatter.
Extract a `_hermes_meta()` helper that returns the nested dict only when it
really is one. Fixes the whole class, not just the two call sites.
Assemble a per-profile graph of memories and learned skills over time
(agent/learning_graph.py) and serve it at GET /api/learning/graph
(hermes_cli/web_server.py), with tests. The radial time axis the desktop
renders is derived from this payload; the REST path stays under /learning
for backend compatibility.
