Docs: Update Agent Skills documentation (#22388)

Co-authored-by: Sam Roberts <158088236+g-samroberts@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

docs/cli/skills-best-practices.md

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+# Agent Skill best practices
+
+Create high-quality, reliable Agent Skills by following these established design
+principles and patterns.
+
+## Design for discovery
+
+The most important part of a skill is its `description`. This is the only
+information the model has before activation.
+
+- **Be specific**: Use keywords that are likely to appear in user prompts (for
+  example, "audit," "security," "refactor," "migration").
+- **Define the trigger**: Clearly state _when_ the skill should be used (for
+  example, "Use this skill when the user asks to review a PR for performance
+  regressions").
+- **Avoid overlap**: Ensure your skill descriptions are distinct from one
+  another and from the general capabilities of the model.
+
+## Progressive disclosure
+
+The "context window" is a shared resource. Use a three-level loading system to
+manage context efficiently.
+
+1.  **Metadata (name + description)**: Always in context (~100 words).
+2.  **`SKILL.md` body**: Loaded only after the skill triggers (<5k words).
+3.  **Bundled resources**: Loaded only as needed by the model.
+
+**Best practice**: Keep the `SKILL.md` body focused on core procedural
+instructions. Move detailed reference material, schemas, and examples into
+separate files in a `references/` directory.
+
+## Degrees of freedom
+
+Match the level of instruction specificity to the task's fragility.
+
+- **High freedom (text-based instructions)**: Use when multiple approaches are
+  valid or decisions depend heavily on context.
+- **Medium freedom (pseudocode or scripts with parameters)**: Use when a
+  preferred pattern exists but some variation is acceptable.
+- **Low freedom (specific scripts, few parameters)**: Use when operations are
+  fragile and error-prone, or a specific sequence MUST be followed.
+
+## Bundle resources effectively
+
+Leverage the skill's ability to include scripts and assets to extend the agent's
+capabilities.
+
+- **Use scripts for deterministic tasks**: If a task can be automated with a
+  script (for example, running a linter, fetching data from an API), bundle it
+  in the `scripts/` folder.
+- **Agentic ergonomics**: Ensure scripts output LLM-friendly stdout. Suppress
+  verbose tracebacks and provide clear, concise success/failure messages.
+- **Provide templates**: Include common file headers or boilerplate code in the
+  `assets/` folder to ensure the agent produces consistent output.
+
+## Anatomy of a great skill
+
+A well-structured skill directory organizes its resources into specialized
+sub-folders.
+
+```text
+my-skill/
+├── SKILL.md       (Required) Core instructions and metadata
+├── scripts/       (Optional) Executable logic (Node.js, Python, etc.)
+├── references/    (Optional) Documentation to be loaded as needed
+└── assets/        (Optional) Templates and non-executable resources
+```
+
+## Security and privacy
+
+Design your skills with security in mind to protect your workspace and data.
+
+- **Avoid hardcoded secrets**: Never include API keys or passwords in your
+  skill's scripts or documentation.
+- **Review third-party skills**: Inspect the `SKILL.md` and scripts of any skill
+  before installing it from an untrusted source.
+- **Limit scope**: Design skills to be as focused as possible to minimize the
+  potential impact of errors.