feat(install): level-aware casan init + merge-safe adoption + hub guardrail

Answers the 3 adoption questions (Plan-21 follow-up):

1) LEVEL SELECTION (4 packaging levels, packaging/levels.json):
   - install.sh --level core|devkit; platform refused (preview service),
     enterprise refused (future). Level recorded in .casan-level.
   - casan init --level 1..4: L1=gate+Plan-20 hooks only; L2=+CI+domain-pack;
     L3=L2 base+preview note; L4=refused. New `casan level show|set`.
   - levels.json core now includes adapters/ + schemas/ + install scripts.

2) EXISTING SHELLS (agents/skills): init MERGES Plan-20 hooks into an existing
   .claude/settings.json and .codex/{hooks.json,config.toml} idempotently
   instead of clobbering — preserves the project's own hooks/agents/skills and
   unrelated keys. Re-running never duplicates the CASAN hook.

3) NO RE-INDEX / NO SHELL REWRITE: init only adds config; it does not parse or
   index code and does not rewrite the project shell.

Safety fixes after a test accidentally ran init in the real repo:
   - launcher shim now SELF-LOCATES its install from its own path (no ambient
     CASAN_HOME cross-talk).
   - casan init REFUSES to adopt a CASAN source hub into itself (--force to
     override), so the Plan-20 hooks can't block the developing agent.
   - test always runs init inside throwaway dirs; +source-hub guard test.

hybrid-install-tests.sh: 41/41 PASS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/levels.json

@@ -19,9 +19,14 @@
         "packages/casan-harness/memory",
         "packages/casan-harness/templates",
         "packages/casan-harness/tests",
+        "packages/casan-harness/adapters",
+        "packages/casan-harness/schemas",
         "packages/casan-harness/init-options.json",
         "packages/casan-harness/README.md",
         "bin/casan",
+        "install.sh",
+        "install.ps1",
+        "packaging/levels.json",
         "VERSION"
       ],
       "excludes_globs": ["**/__pycache__/**", "**/*.pyc", "**/.DS_Store"],