docs: compound learning on release-time consistency-test cascade failures

Documents the cascade pattern surfaced during this session's install-modernization
arc: a `test_sync_cache_path_uses_skill_version` test asserted that a hardcoded
version pin in `sync.sh` matched the version frontmatter in SKILL.md. When a
release bumped SKILL.md, every open PR's CI failed simultaneously on the
unrelated stale-pin assertion. Affected at least 5 PRs across the 2026-05-13
to 2026-05-15 window (#400, #390, #392, and two others) plus required hotfix
PR #397 to unblock the queue.

The permanent fix shipped in PR #405 (deleted sync.sh + the test). This doc
captures the design lesson so the pattern doesn't reappear: don't write
consistency tests that read two files and assert one matches a value derived
from the other. Either derive at runtime from a single source of truth, or
self-skip / merge-base-scope the test so deletion is a non-event.

Created via /ce-compound. Includes:

- docs/solutions/workflow-issues/release-consistency-test-cascade-2026-05-16.md
  (the new learning — first entry under docs/solutions/)
- CONCEPTS.md (new — 4 entries: Skill, Engine, Harness, Beta channel,
  capturing project-specific vocabulary that surfaced across the session)
- AGENTS.md (added one-line Structure entries surfacing docs/solutions/ and
  CONCEPTS.md so fresh agents discover them)
- docs/plans/2026-04-22-{002,003,005,006}-*-plan.md (added deprecation banner
  to each, pointing readers at PR #405 and the new docs/solutions entry —
  these 4 historical plans still reference the deleted sync.sh inline)

Also: closed PR #379 (j-sperling's workaround for the same cascade,
superseded by PR #405's permanent fix).

AGENTS.md

@@ -7,6 +7,8 @@ Agent Skills package for researching any topic across Reddit, X, YouTube, and we
 - `skills/last30days/scripts/last30days.py` — main research engine
 - `skills/last30days/scripts/lib/` — search, enrichment, rendering modules
 - `skills/last30days/scripts/lib/vendor/bird-search/` — vendored X search client
+- `docs/solutions/` — documented solutions to past problems (bugs, best practices, workflow patterns), organized by category with YAML frontmatter (`module`, `tags`, `problem_type`)
+- `CONCEPTS.md` — shared domain vocabulary (Skill, Engine, Harness, Beta channel) — relevant when orienting to the codebase or discussing project terminology
 
 ## Orientation
 - This is an Agent Skills package, not a CLI tool. The product is the slash-command-invoked skill (`/last30days <topic>` in most harnesses); `scripts/last30days.py` is implementation. Claude Code is the most common host but not the only one — features must work across every harness the skill installs into.