docs: add hooks guide covering permissions, auditing, and notifications (#720)

* docs: add image input guide

Add docs/guides/image-input.md documenting how to send images/visual
input to Copilot sessions via file attachments. Covers:

- Quick start examples in all 4 SDK languages (TS, Python, Go, .NET)
- Supported image formats (JPG, PNG, GIF, and other common types)
- Automatic image processing by the runtime (resizing, quality reduction)
- Vision model capability fields for checking support
- Receiving image results from tool execution content blocks
- Practical tips and limitations

All code blocks verified via docs validation. All claims cross-checked
against SDK source code.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* docs: add hooks guide covering permissions, auditing, and notifications

Add a use-case-oriented guide for session hooks that covers:
- Permission control (allow-lists, directory restrictions, ask-before-destructive)
- Auditing & compliance (structured audit logs, secret redaction)
- Notifications & sounds (desktop notifications, Slack webhooks, system sounds)
- Prompt enrichment (project metadata injection, shorthand expansion)
- Error handling & recovery (retry logic, friendly messages)
- Session metrics (duration, tool call counts, end reasons)

Includes multi-language examples (TypeScript, Python, Go, .NET) and links
to the existing API reference docs in docs/hooks/.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: add docs-validate skip for Python snippet in streaming-events guide

The Python subscription example uses 'session' without defining it,
which fails mypy validation in CI. Add skip directive to match the
Go and .NET snippets.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: skip docs validation for Python aiofiles example

The audit log example imports aiofiles which lacks type stubs in the
validation environment.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: patniko