chore: remove legacy per-platform charts and stale build artifacts (closes #70) (#83)

The chart unification (4 per-platform charts -> unified client/ chart)
shipped in v1.1.0; the unified chart has now been at v1.2.x in production
across stg + hasan-prod for several releases. Time to retire the legacy
artifacts.

Removed:
- aks/, bm/, eks/, oc/ chart directories — 75 files, ~330KB. Each had a
  DEPRECATED.md pointing at the unified chart for ~6 months.
- 7 stale .tgz tarballs at repo root (aks-1.0.3, aks-1.0.4, bm-1.0.3,
  bm-1.0.4, eks-1.0.3, eks-1.0.4, oc-1.0.4). The release workflow
  publishes via gh-pages; these checked-in builds were dead weight.
- Root index.yaml — stale snapshot listing only 1.0.3/1.0.4 of the
  legacy charts. The live index served at tracebloc.github.io/client
  is on the gh-pages branch and is the source of truth.
- mysql.yaml at repo root — orphaned PVC manifest with hardcoded volume
  UUID and namespace. Audited: zero references anywhere in the repo.

Other:
- Added *.tgz to .gitignore so chart packages don't sneak back in.
- Updated client/MIGRATION.md Rollback section. The old "the legacy
  charts remain in aks/, bm/, eks/, oc/ and can be used at any time"
  was about to become a lie. Replaced with instructions to recover the
  directory from git history if anyone genuinely needs the old chart.

Verification:
- helm lint --strict ./client -f client/ci/eks-values.yaml — clean
  (same invocation the release workflow runs on every tag)
- helm unittest client — 105/105 still passing
- helm package ./client -d /tmp — produces a valid client-1.2.3.tgz

Net diff: 86 files changed, 17 insertions(+), 3447 deletions(-).

Co-authored-by: Lukas Wuttke <lukas@tracebloc.io>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: lukasWuttke <54042461+LukasWodka@users.noreply.github.com>

Author: 3 people

.gitignore

@@ -132,4 +132,9 @@ value_files/
 
 # Tenant migration tools — populated config carries Docker Hub PAT and
 # per-tenant CLIENT_PASSWORD. Only the .example.env template is committed.
-docs/migration-tools/tenant-config.env
+docs/migration-tools/tenant-config.env
+
+# Helm chart packages. Releases are published from gh-pages by the
+# release-helm-chart.yaml workflow; checked-in .tgz files are dead weight
+# (and were the cause of the cleanup in #70).
+*.tgz