fix: remove model field from lettabot config, add lettabot model command

The model field in lettabot.yaml was redundant and misleading -- the model
is configured on the Letta agent server-side, and lettabot shouldn't
override it. Users were confused seeing a model in their startup log that
didn't match the actual model being used.

Changes:
- Remove `model` from `LettaBotConfig.agent` (made optional for backward compat)
- Remove `model` from `BotConfig` interface and `bot.ts` createAgent() calls
- Remove `model` from `main.ts` config construction and startup log
- Stop saving `model` to lettabot.yaml during onboarding
- Stop mapping `agent.model` to `MODEL` env var in config/io.ts
- Add `getAgentModel()` and `updateAgentModel()` to letta-api.ts
- Add new `src/commands/model.ts` with three subcommands:
  - `lettabot model` - interactive model selector
  - `lettabot model show` - show current agent model
  - `lettabot model set <handle>` - set model directly
- Wire up model command in cli.ts with help text
- Update docs/configuration.md, lettabot.example.yaml, SKILL.md

Model selection during `lettabot onboard` is preserved for new agent
creation -- the selected model is passed to createAgent() but is NOT
saved to the config file.

Fixes #169

Co-authored-by: Cameron <cpfiffer@users.noreply.github.com>

SKILL.md

@@ -35,7 +35,7 @@ lettabot server
 **Safe defaults used if not set:**
 - `LETTA_BASE_URL`: `https://api.letta.com`
 - `LETTA_AGENT_NAME`: `"lettabot"`
-- `LETTA_MODEL`: `"claude-sonnet-4"`
+- Model: selected during `lettabot onboard` or changed with `lettabot model set <handle>`
 - `*_DM_POLICY`: `"pairing"` (requires approval before messaging)
 - `WHATSAPP_SELF_CHAT_MODE`: `true` (only "Message Yourself" chat)
 - `SIGNAL_SELF_CHAT_MODE`: `true` (only "Note to Self")
@@ -70,7 +70,7 @@ The wizard will guide you through:
 |----------|-------------|---------|
 | `LETTA_AGENT_ID` | Use existing agent (skip agent creation) | Creates new agent |
 | `LETTA_AGENT_NAME` | Name for new agent | `"lettabot"` |
-| `LETTA_MODEL` | Model for new agent | `"claude-sonnet-4"` |
+| ~~`LETTA_MODEL`~~ | *Removed* - model is set on the agent server-side. Use `lettabot model set <handle>`. | - |
 
 ### Telegram
 
@@ -263,7 +263,7 @@ npm install && npm run build && npm link
 # 4. Set environment variables
 export LETTA_API_KEY="letta_..."
 export TELEGRAM_BOT_TOKEN="123456:ABC-DEF..."
-# Defaults will be used for LETTA_BASE_URL, agent name, model, and DM policy
+# Defaults will be used for LETTA_BASE_URL, agent name, and DM policy
 
 # 5. Run non-interactive setup
 lettabot onboard --non-interactive