v2.4.1 — compaction passes II–III

Maintenance/refactor pass — zero behavior change (validator-attested: skill-frontmatter, agent-frontmatter, markdown-link, check-registry, reference-compression, plugin-structure, count-sync, version-sync all exit 0; hook test suite 66/66). Skill semantics, hook wiring, agent roles, and flags are identical. Attacks the content pools v2.4.0 left untouched (docs/, references/main.md), plus changelog history and duplicated hook helpers.

Tracked lines 54,803 → 42,301 (−12,502).

• docs/ 91 → 14 files (11,408 → 1,854 lines). Removed the uncited 51-file docs/validation/v1.16.0/ snapshot (recoverable via git tag validation-v1.16.0) and archived design-process history across consolidation/review-audit, integrations/harness-design, integrations/impeccable, security/containment — keeping only runtime-cited references (effectiveness-research.md, audit-spec.md, design-critic-upgrade.md, references-regrounded.md, detector-rebuild.md, blitz-surface-map.md, the impeccable design pillar core + Apache-2.0 license/attribution). Every inbound link to a removed file rewritten in-commit.
• .review/ untracked + gitignored — audit-skill generated output, not source (same category as the .original files v2.4.0 removed).
• counts.json phrasing fix — canonical_phrasing.shared now reads "12 shared protocol files".
• references/main.md (12,718 → 12,708) — collapsed two duplicated file-lock step-lists in roadmap/references to cite session-lifecycle.md §File-Based Locking Protocol. The hypothesized large de-boilerplating win did not materialize: the top reference files are dense skill-specific procedure protected by named-section SKILL.md contracts and the agent-prompt-payload invariant (intentional verbatim duplication), not boilerplate.
• agent-orchestration.md (1,454 → 1,435) — removed two HEARTBEAT/PARTIAL default blocks re-embedded in the boilerplate section, which already declared §3 canonical. No _shared file re-split.
• CHANGELOG.md (870 → 201 lines, −87 KB) — archived the 16 1.x releases (1.16.0 → 1.5.0) to CHANGELOG-ARCHIVE.md behind an "Older releases" pointer; kept [Unreleased], the Release-Process header, and all 2.x releases live so version-sync stays green.
• hooks/scripts/_lib/common.sh — hoisted the byte-identical fail() helper out of agent-frontmatter-validate.sh + skill-frontmatter-validate.sh (both already sourced common.sh). find_project_root/block/validate_one/usage left inline — not byte-identical across call sites (or semantically distinct from blitz_find_root). Hook-script count unchanged (38).