feat: add LETTABOT_CONFIG env var for config path (#152)

* docs: add TESTING.md guide

Comprehensive testing documentation covering:
- Unit test setup and patterns
- E2E test setup with Letta Cloud
- MockChannelAdapter usage
- CI/CD workflow
- Best practices

Written by Cameron ◯ Letta Code

* feat: add LETTABOT_CONFIG env var for config path

Addresses Discord feedback from fpl9000 who was confused about where
to put the config file after a global npm install.

Changes:
- Add LETTABOT_CONFIG env var that overrides the config search order
- Update error messages to show the env var option
- Document in docs/configuration.md

Now users doing global installs can either:
- Create ~/.lettabot/config.yaml, or
- Set LETTABOT_CONFIG=/path/to/config.yaml

Written by Cameron ◯ Letta Code

"Configuration should be explicit, not magic." - The Twelve-Factor App

docs/configuration.md

@@ -6,10 +6,15 @@ Complete reference for LettaBot configuration options.
 
 LettaBot checks these locations in order:
 
-1. `./lettabot.yaml` - Project-local (recommended)
-2. `./lettabot.yml` - Project-local alternate
-3. `~/.lettabot/config.yaml` - User global
-4. `~/.lettabot/config.yml` - User global alternate
+1. `LETTABOT_CONFIG` env var - Explicit path override
+2. `./lettabot.yaml` - Project-local (recommended)
+3. `./lettabot.yml` - Project-local alternate
+4. `~/.lettabot/config.yaml` - User global
+5. `~/.lettabot/config.yml` - User global alternate
+
+For global installs (`npm install -g`), either:
+- Create `~/.lettabot/config.yaml`, or
+- Set `export LETTABOT_CONFIG=/path/to/your/config.yaml`
 
 ## Example Configuration
 
@@ -203,6 +208,7 @@ Environment variables override config file values:
 
 | Env Variable | Config Equivalent |
 |--------------|-------------------|
+| `LETTABOT_CONFIG` | Path to config file (overrides search order) |
 | `LETTA_API_KEY` | `server.apiKey` |
 | `LETTA_BASE_URL` | `server.baseUrl` |
 | `LETTA_AGENT_ID` | `agent.id` |