[feature] Add /add-dir command to bulk-add a directory to session context

## Problem

There is no command to bulk-add all files from a directory to the agent's context. Users must reference files individually or hope the agent discovers them through exploration. When working with an entire module, data pipeline, or dbt model directory, this is tedious.

## Proposed solution

Add an `/add-dir` slash command:

```
/add-dir path/to/dir # add all files in directory
/add-dir models/staging --depth 1 # add files, limit recursion depth
/add-dir src/ --include "*.sql" # add only matching files
```

The command should:
1. Recursively enumerate files in the given directory
2. Respect `.gitignore` and common ignore patterns
3. Add file contents to the agent's context window
4. Report how many files / tokens were added
5. Warn if the addition would exceed context limits

## Current workarounds

- Manually referencing files one by one
- Hoping the agent uses `glob` or `grep` to discover relevant files
- Using `Read` tool on individual files

## Additional context

- Built-in commands today are: `/init`, `/discover`, `/review`, `/feedback`, `/configure-claude`, `/configure-codex`
- Custom commands can be defined in `.altimate-code/commands/` but this is a common enough need to be built-in