Migrated product requirements and design documentation from RST to Openspec Markdown format as per instructions.

Changes:

1. PRD Migration:

   • Analyzed documentation/prd.rst.
   • Created capability specs in documentation/architecture/openspec/specs/ for:
     • mimetype-detection
     • charset-detection
     • line-separator-processing
     • text-validation
     • api (identified as a cross-cutting capability for API/behavior designs)
   • Defined Requirements and Scenarios (required for Openspec validation).

2. Design Migration:

   • Converted documentation/architecture/designs/00*.rst to Markdown using rst2myst.
   • Mapped designs to capabilities:
     • 001, 002, 003 -> api/design.md
     • 004 -> charset-detection/design.md
   • Performed editorial cleanup (demoted headers to fit document structure).

3. Cleanup:

   • Removed original RST files (prd.rst, designs/00*.rst).
   • Updated documentation/architecture/designs/index.rst to point to the new designs.

Validation:

• Ran openspec validate --specs --strict - All passed.
• Ran hatch --env develop run docsgen - Build successful (orphan warnings resolved).

PR created automatically by Jules for task 5263045212488424178 started by @emcd