Add comprehensive README.md to project root#

Problem#

The repo had a single-line placeholder README providing no useful context for engineers or agents discovering it for the first time.

Task / Link#

Closes #676

Changes#

• Replaced one-line placeholder README with a full documentation page
• Covers repo purpose and "not a service" note
• Documents directory structure (docs/ and tasks/) with a tree view
• Explains naming conventions for task directories
• Adds a table describing the role of each task subdirectory (plan/, meta/, input/, changes/)
• Includes a minimal dev workflow / branch & PR rules section

Notes#

Directory structure in the README is manually maintained and may drift from the actual layout over time — acceptable per issue scope.

Testing#

• Verified README exists at repo root and exceeds 20 lines
• Content derived from authoritative CLAUDE.md; no build system or tests to run in this repo