refactor(linkedin-studio): move agent fasit fixtures + tests out of agents/ (v0.5.3)

The 6 long-form-review fasit fixtures (agents/fixtures/*-cases.md) were registered
by Claude Code as junk `fixtures:*` agents because agents/ is scanned recursively.
They are test fixtures, not capabilities. Moved them + their 6 lint tests to
tests/ + tests/fixtures/ and retargeted the 5 agent fasit-refs. Count-neutral
namespace hygiene (ls agents/*.md still 19).

- agents/fixtures/*-cases.md            -> tests/fixtures/        (6)
- agents/__tests__/*-fixture.test.mjs   -> tests/                 (6; ../fixtures -> ./fixtures)
- 5 agent refs ${CLAUDE_PLUGIN_ROOT}/agents/fixtures -> /tests/fixtures
- version-sync 0.5.2 -> 0.5.3 (plugin.json / README badge / CLAUDE.md / CHANGELOG)

Verify: fixture-lint 35/35 at new path; render 20/20, hooks 139/139; test-runner 109/0/0.
Reinstall/reload required for the 6 spurious agents to leave the registry.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CrTb8ktf1XZWEVwgz5MTTo

.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "linkedin-studio",
-  "version": "0.5.2",
+  "version": "0.5.3",
   "description": "LinkedIn Studio — full-spectrum LinkedIn content engine: feed posts, carousels, video scripts, and long-form newsletter editions, with the 2026 relevance-ranking model baked in. v4.0.0 is an audit-remediation release (Voyage Phase 0–3): every user-facing claim is made honest or removed, all 11 previously-orphaned agents are wired (→ 19 agents), a `/linkedin:firsthour` post-publish command is added (→ 27 commands), the algorithm-signal claims are reconciled to one sourced statement (no unpublishable model name or date), short-form de-AI and video quality gates are added, and the structure lint is rebuilt to guard the real layout plus version/count/stat consistency. Breaking: the newly-wired agents register only on reinstall/reload, and this consolidates the v3.0.0 identity break (slug, agent namespace `linkedin-studio:<agent>`, state-file path `~/.claude/linkedin-studio.local.md`). v3.1.0 added the cold adversarial review package (`/linkedin:headless-review` + Step 6.5 + `/linkedin:pivot` + per-artifact personas); the `/linkedin:*` commands are unchanged. v4.1.0 adds a journey layer: two guided front-doors (`/linkedin:create`, `/linkedin:measure`) plus a router re-tiered into five journeys (Start · Create · Engage · Measure · Grow), with the 27 existing commands kept as the execution tier (→ 29 commands; additive, reload registers the two new commands).",
   "author": {
     "name": "Kjell Tore Guttormsen"