feat(skill): align skill system with Claude Code's Tool Call design

- Replace text marker activation with `activate` tool (Tool Call mechanism)
- Replace 7 authoring tools with 2: `activate` + `register_skill`
- Add builtin skills loading from templates/skills/
- Add create-skill as first builtin skill
- Remove SKILL_ACTIVATION_MARKER and text detection methods
- Tool Result returns SKILL.md content (protects KV Cache)

This aligns with Claude Code's progressive disclosure pattern:
- Metadata (name+description) always visible in tool description
- SKILL.md body loaded on activate via Tool Call
- Bundled resources accessible through virtual path mapping

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/langbot/templates/skills/create-skill/SKILL.md

@@ -0,0 +1,104 @@
+---
+name: create-skill
+description: Create new skills for LangBot. Use when users want to create, modify, or register a skill. Helps draft SKILL.md with proper frontmatter and register skills from sandbox directories.
+---
+
+# Create Skill
+
+A skill for creating new LangBot skills.
+
+## Skill Structure
+
+A skill is a directory containing, at minimum, a `SKILL.md` file:
+
+```
+skill-name/
+├── SKILL.md          # Required: metadata + instructions
+├── scripts/          # Optional: helper scripts
+├── references/       # Optional: documentation
+├── assets/           # Optional: templates, resources
+```
+
+## SKILL.md Format
+
+SKILL.md must contain YAML frontmatter followed by Markdown content:
+
+```markdown
+---
+name: skill-name
+display_name: Skill Display Name
+description: What this skill does and when to use it.
+---
+
+# Skill Title
+
+Instructions for how to use this skill...
+
+## Examples
+
+- Example 1
+- Example 2
+```
+
+### Frontmatter Fields
+
+| Field | Required | Description |
+|-------|----------|-------------|
+| `name` | Yes | Skill identifier. 1-64 chars, lowercase letters, numbers, hyphens only. Must not start or end with hyphen. |
+| `display_name` | No | Human-readable name shown in UI. |
+| `description` | Yes | What the skill does and when to use it. Max 1024 chars. Include keywords for triggering. |
+| `license` | No | License name or reference. |
+| `metadata` | No | Additional key-value metadata. |
+
+### Body Content
+
+The Markdown body contains skill instructions. Recommended sections:
+- Step-by-step instructions
+- Examples
+- Common edge cases
+- Guidelines
+
+Keep SKILL.md under 500 lines. Move detailed content to `references/` directory.
+
+## Creating a Skill
+
+1. **Understand requirements**: Ask what the skill should do
+2. **Draft SKILL.md**: Create frontmatter + instructions
+3. **Create in sandbox**: Write files to `/workspace/{skill-name}/`
+4. **Register**: Use `register_skill` tool to register to LangBot store
+
+### Name Rules
+
+- Lowercase letters, numbers, hyphens only
+- Cannot start or end with hyphen
+- No consecutive hyphens (`--`)
+- 1-64 characters
+
+Valid: `pdf-processing`, `data-analysis`, `code-review`
+Invalid: `PDF-Processing`, `-pdf`, `pdf--processing`
+
+### Description Tips
+
+Good description describes both what and when:
+```yaml
+description: Extracts text from PDF files. Use when working with PDF documents or when user mentions PDFs.
+```
+
+Poor description:
+```yaml
+description: Helps with PDFs.
+```
+
+## Workflow Example
+
+1. User: "Create a skill for generating reports"
+2. Ask clarifying questions about report format, templates, etc.
+3. Create `/workspace/report-generator/SKILL.md`
+4. Optionally create helper scripts in `/workspace/report-generator/scripts/`
+5. Call `register_skill(path="/workspace/report-generator", name="report-generator")`
+6. Skill is now available via `activate(skill_name="report-generator")`
+
+## After Registration
+
+The skill package is copied to `data/skills/` and loaded by LangBot.
+User can activate it immediately with the `activate` tool.