docs(conventions): group thousands with a comma (phaze-xp9nx)

[SimplicityGuy]

Summary

Adds a new section to CONVENTIONS.md, next to "Replace identifiers, never quantities", stating the number-grouping convention for tracked prose: comma grouping (4,383), never space grouping — including four-digit numbers.

What it covers

• The rule: comma groups the integer part; four-digit numbers are grouped too (4,383, never 4383); decimals keep the comma on the integer part and the period as the decimal point (4,761.835).
• What is never grouped: years, ports, bead ids, version numbers, line numbers, SHAs — any identifier that merely looks numeric. Cites the PERF-02 200K false positive in .planning/milestones/2026.7.5-ROADMAP.md to show this is a judgement call about meaning, not a mechanical pattern (and why no grep/lint guard is added).
• Scope: tracked prose (docs/**, root-level *.md, spike/design docs, planning notes, commit messages, PR bodies) — not source code.
• The measured evidence for why the rule is needed: the corpus already splits by genre (measurement spikes space-grouped, diagnosis spikes and everything else comma-grouped), and how phaze-zaf2l's spike forked because it matched space-grouped exemplars with no written rule to check against.
• The operator decision (question-as-put, answer-as-given, durable record) that authorizes writing the rule down, and why the answer is comma grouping.
• A note that the existing space-grouped corpus (~440 matches across 44 files) is being converted under the companion bead phaze-3x7xt, not silently left inconsistent.

Out of scope (deliberately)

• No existing document's numbers are converted here — that's phaze-3x7xt.
• No mechanical grep/lint guard is added — an automated rewrite-on-pattern-match would reproduce the PERF-02 200K false positive.

Validation

just check-fast — green: ruff clean, mypy clean, docs-selector correctly picked the 11 prose-guard modules touching CONVENTIONS.md (including tests/shared/test_adr_citation_resolution.py and tests/shared/test_operator_attribution_citations.py), 194 passed.

Bead: phaze-xp9nx

🤖 Generated with Claude Code