docs: document all three import path resolution modes

[Copilot]

Users didn't know .github/agents/ imports were failing, that the .github/-prefix syntax existed after the fix, or that cross-repo imports were already supported. The imports reference and JSON schema only described the owner/repo/path@ref cross-repo form.

Changes

docs/src/content/docs/reference/imports.md

• Replaced the one-liner "Path Formats" section with a full reference covering all three resolution modes:

Mode	Syntax	Resolved against
Relative (default)	shared/file.md	.github/workflows/
Repo-root-relative	.github/agents/file.md or /path/file.md	Repository root
Cross-repo	owner/repo/path@ref	Remote GitHub repository

• Added per-mode subsections with examples, a worked example showing all three in one frontmatter block, a backward-compatibility note, and security constraints (.github/ boundary enforcement).

pkg/parser/schemas/main_workflow_schema.json

• Updated imports top-level description and all string/path/uses item descriptions to enumerate all three path forms — previously only the cross-repo owner/repo/path@ref form was mentioned.

Example: all three forms in one workflow

imports:
  # 1. Relative — resolved from .github/workflows/
  - shared/common-tools.md
  # 2. Repo-root-relative — resolved from repository root
  - .github/agents/code-reviewer.md
  # 3. Cross-repo — pinned to a release tag
  - acme-org/shared-workflows/mcp/tavily.md@v1.0.0