feat: add acquiring-skills (#466)

Co-authored-by: Letta <noreply@letta.com>

src/skills/builtin/acquiring-skills/SKILL.md

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+---
+name: acquiring-skills
+description: Guide for safely discovering and installing skills from external repositories. Use when a user asks for something where a specialized skill likely exists (browser testing, PDF processing, document generation, etc.) and you want to bootstrap your understanding rather than starting from scratch.
+---
+
+# Acquiring New Skills
+
+This skill teaches you how to safely discover and install skills from external sources.
+
+## SAFETY - READ THIS FIRST
+
+Skills can contain:
+- **Markdown files** (.md) - Risk: prompt injection, misleading instructions
+- **Scripts** (Python, TypeScript, Bash) - Risk: malicious code execution
+
+### Trusted Sources (no user approval needed for download)
+- `https://github.com/letta-ai/skills` - Letta's community skills
+- `https://github.com/anthropics/skills` - Anthropic's official skills
+
+### Untrusted Sources (ALWAYS verify with user)
+For ANY source other than letta-ai or anthropics:
+1. Ask the user before downloading
+2. Explain where the skill comes from
+3. Get explicit approval
+
+### Script Safety
+Even for skills from trusted sources, ALWAYS:
+1. Read and inspect any scripts before executing them
+2. Understand what the script does
+3. Be wary of network calls, file operations, or system commands
+
+## When to Use This Skill
+
+**DO use** when:
+- User asks for something where a skill likely exists (e.g., "help me test this webapp", "generate a PDF report")
+- You think "there's probably a skill that would bootstrap my understanding"
+- User explicitly asks about available skills or extending capabilities
+
+**DON'T use** for:
+- General coding tasks you can already handle
+- Simple bug fixes or feature implementations
+- Tasks where you have sufficient knowledge
+
+## Ask Before Searching (Interactive Mode)
+
+If you recognize a task that might have an associated skill, **ask the user first**:
+
+> "This sounds like something where a community skill might help (e.g., webapp testing with Playwright). Would you like me to look for available skills in the Letta or Anthropic repositories? This might take a minute, or I can start coding right away if you prefer."
+
+The user may prefer to start immediately rather than wait for skill discovery.
+
+Only proceed with skill acquisition if the user agrees.
+
+## Skill Repositories
+
+| Repository | Description |
+|------------|-------------|
+| https://github.com/letta-ai/skills | Community skills for Letta agents |
+| https://github.com/anthropics/skills | Anthropic's official Agent Skills |
+
+Browse these repositories to discover available skills. Check the README for skill listings.
+
+## Installation Locations
+
+| Location | Path | When to Use |
+|----------|------|-------------|
+| **Global** | `~/.letta/skills/<skill>/` | General-purpose skills useful across projects |
+| **Project** | `.skills/<skill>/` | Project-specific skills |
+
+**Rule**: If useful across multiple projects, install globally. If project-specific, install in `.skills/`.
+
+## How to Download Skills
+
+Skills are directories containing SKILL.md and optionally scripts/, references/, examples/.
+
+### Method: Clone to /tmp, then copy
+
+```bash
+# 1. Clone the repo (shallow)
+git clone --depth 1 https://github.com/anthropics/skills /tmp/skills-temp
+
+# 2. Copy the skill to your skills directory
+# For global:
+cp -r /tmp/skills-temp/skills/webapp-testing ~/.letta/skills/
+# For project:
+cp -r /tmp/skills-temp/skills/webapp-testing .skills/
+
+# 3. Cleanup
+rm -rf /tmp/skills-temp
+```
+
+### Alternative: rsync (preserves permissions)
+
+```bash
+git clone --depth 1 https://github.com/anthropics/skills /tmp/skills-temp
+rsync -av /tmp/skills-temp/skills/webapp-testing/ ~/.letta/skills/webapp-testing/
+rm -rf /tmp/skills-temp
+```
+
+## Registering New Skills
+
+After downloading, refresh the skills list:
+
+```
+Skill(command: "refresh")
+```
+
+This scans `~/.letta/skills/` and `.skills/` and updates your `skills` memory block.
+
+## Complete Example
+
+User asks: "Can you help me test my React app's UI?"
+
+1. **Recognize opportunity**: Browser/webapp testing - likely has a skill
+2. **Ask user**: "Would you like me to look for webapp testing skills, or start coding right away?"
+3. **If user agrees, find skill**: Check anthropics/skills for webapp-testing
+4. **Download** (trusted source):
+   ```bash
+   git clone --depth 1 https://github.com/anthropics/skills /tmp/skills-temp
+   cp -r /tmp/skills-temp/skills/webapp-testing ~/.letta/skills/
+   rm -rf /tmp/skills-temp
+   ```
+5. **Refresh**: `Skill(command: "refresh")`
+6. **Inspect scripts**: Read any .py or .ts files before using them
+7. **Load**: `Skill(command: "load", skills: ["webapp-testing"])`
+8. **Use**: Follow the skill's instructions for the user's task