docs: README overhaul + documentation restructure

[dacervera]

What changed

README.md rewritten as a focused landing page (~120 lines, down from ~400):

• Kept the 1st-person Quorum voice and compelling narrative
• Quick demo → comparison table → installation options
• Platform ports section (Copilot CLI, Claude Code)
• Model tiers (trimmed)
• Documentation table linking to organized docs

Documentation reorganized by intent:

Directory	Contents
docs/getting-started/	Installation, Quick Start, For Beginners, Tutorial, Model Requirements
docs/configuration/	Config Reference
docs/guides/	Rubric Building Guide, Cross-Artifact Design
docs/architecture/	The Nine Critics, Implementation Notes
docs/critics/	Security, Code Hygiene, Tester, SEC-02 through SEC-07, PowerShell
docs/reviews/	External Reviews

New files:

• docs/getting-started/QUICK_START.md — first validation in 60 seconds
• docs/README.md — navigation hub with categorized links

All internal cross-references updated across 30+ files. Git rename tracking preserved for all moved files.

[dacervera]

Docs restructure + README rewrite. 23 docs reorganized into 6 categorized subdirectories. 7 broken relative links found and fixed during Quorum self-validation. 2 pre-existing stale references corrected (SKILL.md critic count, README version badge). validate-docs.py exclusion updated for new docs/reviews/ path. Version bumped to v0.7.2.

Quorum Self-Validation: 3 HIGH findings accepted (false positives) on validate-docs.py

• HIGH (completeness, PC-002): "read_file_lines returns None but callers don't check" — False positive. Both call sites in validate_docs() and main() explicitly check if lines is None: continue.
• HIGH (correctness, PC-004): "read_file_lines returns None on size check, inconsistent behavior" — Intentional design. Documented in docstring: "Skips files exceeding MAX_FILE_SIZE_BYTES." Callers handle None.
• HIGH (correctness, PC-012): "External file input without content validation" — Not applicable. Developer CLI tool reading known repo files via read_text().splitlines(). No eval/exec/untrusted parsing.

3 MEDIUM (function length, code duplication, docstring mismatch) and 2 LOW (magic numbers, file handle lifecycle) are pre-existing code quality notes, not introduced by this PR. 3 additional findings excluded by Tester (L1 contradicted).