feat(spec,validate)!: add §20.5 1000-line source file cap with opt-in exception marker

[niclaslindstedt]

Summary

Adds a new mandate to OSS_SPEC.md (§20.5) capping non-test source files at 1000 physical lines, with an opt-in exception via oss-spec:allow-large-file: <reason> in a comment within the file's first 20 lines (non-empty reason required).

• Spec — bumped to v2.3.0; §20 retitled "Source and test organization"; new §20.5 covers the rule, rationale, exception mechanism, and explicit auto-fix scope.
• Validator — new check in src/validate/content.rs walks src//lib/, skips files whose stem matches the §20.2 test regex, and honours the marker only when the reason is non-empty.
• Refactor (breaking internal layout) — src/validate.rs (1151 lines) is split into src/validate/{mod,structural,content,agent_skills,toolchain}.rs so the reference impl exemplifies §20.5 rather than using its own escape hatch. Public re-exports (Report, Violation, AiFinding, run, check_*, version_ge, …) keep the external API stable; internal callers that did use crate::validate::foo continue to work.
• Auto-fix — prompts/fix-conformance bumped to 1.2.0. Guidance: only resolve §20.5 when the file also has a §20 inline-test violation (extracting the test block typically brings it back under the cap); otherwise leave the file alone and surface for manual split or marker annotation.
• Docs synced — README spec badge, AGENTS.md (architecture, "Source file size" section, sync-points), templates/_common/AGENTS.md.tmpl, docs/architecture.md, and the update-spec / sync-oss-spec skill mapping tables (both repo and template versions).

Test plan

• make build — clean
• make test — 76 validator tests pass (70 previous + 6 new: over-limit flags, test-file exempt, marker-with-reason exempts, empty-reason marker doesn't exempt, exact-1000 boundary clean, marker-beyond-line-20 doesn't exempt); self-conformance passes
• make lint — zero clippy warnings
• make fmt-check — applied
• Manual: bootstrap fresh demo project → validate clean; inject 1050-line src/bloat.rs → §20.5 violation with correct message; add marker with reason → clean

Checklist

• Tests added/updated (tests/validate_test.rs + tests/prompts_test.rs)
• Documentation updated:
  • README.md (spec version badge)
  • OSS_SPEC.md (v2.3.0, §20 retitle, §20.5)
  • AGENTS.md + templates/_common/AGENTS.md.tmpl
  • docs/architecture.md
  • .agent/skills/{update-spec,sync-oss-spec}/SKILL.md + template versions
• Commit message follows conventional commit style (feat! — breaking because existing repos with >1000-line files now fail oss-spec validate)