feat: sync content definitions from core repo

Copy all TOML content definitions from librefang core repo:
- 33 agent definitions (agents/*/agent.toml)
- 14 hand definitions with docs (hands/*/HAND.toml + SKILL.md)
- 25 integration templates (integrations/*.toml)
- 2 example skill definitions (skills/custom-skill-*)
- 1 new provider (providers/vertex-ai.toml)

Part of the framework-vs-content registry split (RFC v0.7).

skills/custom-skill-python/skill.toml

@@ -0,0 +1,20 @@
+## Custom Python Skill Example
+##
+## This skill counts words in a given text and returns basic statistics.
+## To test: librefang skill test ./examples/custom-skill-python --input '{"text": "Hello world"}'
+##
+## For the full reference, see: docs/skill-development.md
+
+[skill]
+name = "word-counter"
+version = "0.1.0"
+description = "Count words, sentences, and characters in text."
+author = "your-name"
+tags = ["text", "utility", "example"]
+
+[runtime]
+type = "python"
+entry = "main.py"
+
+[input]
+text = { type = "string", description = "The text to analyze", required = true }