Introduce "agents"

An agent is currently a name given to a system prompt and a set of
tools which the agent has access to.

This resolves the previous issue of the set of configured tools being
available in *all* contexts, which wasn't always desired. Tools are now
only available when an agent is explicitly requested using the
`-a/--agent` flag.

Agents are expected to be expanded on: the concept of task-specilized
agents (e.g. coding), the ability to define a set of files an agent
should always have access to for RAG purposes, etc.

Other changes:

- Removes the "tools" top-level config structure (though this is expected
to come back along with the abillity to define custom tools).

- Renamed `pkg/agent` to `pkg/agents`

TODO.md

@@ -4,15 +4,15 @@
   when calling anthropic?
 - [x] `dir_tree` tool
 - [x] Implement native Anthropic API tool calling
-- [ ] Agents - a name given to a system prompt + set of available tools +
+- [x] Agents - a name given to a system prompt + set of available tools +
   potentially other relevent data (e.g. external service credentials, files for
   RAG, etc), which the user explicitly selects (e.g. `lmcli chat --agent
-  pair-programmer`, `lmcli chat -a financier`).
-    - Specialized agents which have integrations beyond basic tool calling,
+  code-helper`, `lmcli chat -a financier`).
+    - [ ] Specialized agents which have integrations beyond basic tool calling,
       e.g. a coding agent which bakes in efficient code context management
       (only the current state of relevant files get shown to the model in the
       system prompt, rather than having them in the conversation messages)
-    - Agents may have some form of long term memory management (key-value?
+    - [ ] Agents may have some form of long term memory management (key-value?
       natural lang?).
 - [ ] Support for arbitrary external script tools
 - [ ] Search - RAG driven search of existing conversation "hey, remind me of