docs: Update documentation for PR #1054 – prompts field support

[Copilot]

Description

Updates user documentation to reflect the prompts field introduced in PR #1054, which adds support for packaging LLM prompt files in ModelKits.

Kitfile Format Reference (pkg/artifact/kitfile.md)

• Added prompts section documentation with field definitions (path, description)
• Updated overview to include prompts in manifest sections list
• Added example YAML showing prompts usage

CLI Reference (docs/src/docs/cli/cli-reference.md)

• kit init: Documented automatic prompt detection (files containing .prompt, AGENTS.md, SKILL.md, CLAUDE.md)
• kit unpack: Added prompts to valid filter types with examples:

  kit unpack myrepo/my-model:latest --filter=prompts
  kit unpack myrepo/my-model:latest --filter=prompts:system.prompt.md

Kitfile Overview (docs/src/docs/kitfile/kf-overview.md)

• Added prompts to sections table
• Updated minimal Kitfile requirements
• Added prompts to full example YAML

Linked issues

Relates to #1054

AI-Assisted Code

• This PR contains AI-generated code that I have reviewed and tested
• I take full responsibility for all code in this PR, regardless of how it was created

Original prompt

generate documenatation for PR #1054

Custom agent used: user-documentation-agent
Updates user-facing documentation based on code changes in PRs.

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