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.md

@@ -1,4 +1,4 @@
-# LinkedIn Studio Plugin (v0.5.2)
+# LinkedIn Studio Plugin (v0.5.3)
 
 > **Version re-baseline (2026-05-31) · M0 landed (2026-06-18, v0.5.0).** Previously numbered 1.0.0–4.1.0; those were pre-release iterations. The honest maturity is **v0.5.0** — **the architecture workstream (M0) is complete: user data now lives in a per-user data dir (`${LINKEDIN_STUDIO_DATA:-$HOME/.claude/linkedin-studio}/`) resolved through one seam per runtime, with automatic idempotent migration on session-start** (see `references/data-path-convention.md`). No command has yet passed the hardening quality-gate, command testing is incomplete, and there is no GUI. The remaining path to **v1.0.0** is three workstreams: **hardening** (every command through the quality-gate), **command testing**, and a **GUI**. The development narrative below is preserved as history.