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>

install.ps1

@@ -92,7 +92,12 @@ $binDir = Join-Path $CasanHome 'bin'
 New-Item -ItemType Directory -Force -Path $binDir | Out-Null
 $launcherPs1 = @'
 $ErrorActionPreference = "Stop"
-$CasanHome = if ($env:CASAN_HOME) { $env:CASAN_HOME } else { Join-Path $env:LOCALAPPDATA "casan" }
+# Self-locate THIS install from the launcher's own path (<home>\bin\casan.ps1) so a
+# specific launcher always uses ITS install regardless of any ambient CASAN_HOME.
+$selfHome = Split-Path -Parent (Split-Path -Parent $MyInvocation.MyCommand.Path)
+if ((Test-Path (Join-Path $selfHome "current")) -or (Test-Path (Join-Path $selfHome "versions"))) {
+  $CasanHome = $selfHome
+} elseif ($env:CASAN_HOME) { $CasanHome = $env:CASAN_HOME } else { $CasanHome = Join-Path $env:LOCALAPPDATA "casan" }
 $cur = Join-Path $CasanHome "current"
 if (-not (Test-Path $cur)) { Write-Error "casan: no install at $cur (run install.ps1)"; exit 1 }
 $env:CASAN_HARNESS_ROOT = Join-Path $cur "packages\casan-harness"