Add config-as-code coverage for the selftune feature

[JSONbored]

Part of #4092.

Problem

isSelfTuneEnabled (src/review/selftune-wire.ts) is not migrated onto the existing per-repo feature-activation mechanism (src/review/feature-activation.ts — resolveConvergedFeature/convergedFeatureActive, CONVERGED_FEATURE_KEYS, already covers rag/reputation/unifiedComment/safety; NOT a new resolver to build, see #4092's corrected state) — purely GITTENSORY_REVIEW_SELFTUNE + the shared allowlist today.

Fix

• Add "selftune" to CONVERGED_FEATURE_KEYS + FEATURE_GLOBAL_FLAG (mapping to isSelfTuneEnabled).
• Migrate selftune's real call site(s) to convergedFeatureActive/resolveConvergedFeature.
• Given selftune auto-applies tuning changes (shadow-soaked per existing tests), double-check whether per-repo activation has any interaction with the shadow-soak/auto-apply-hold mechanism that needs a specific test.
• .gittensory.yml.example + config/examples/gittensory.full.yml document the new features.selftune key.

Deliverables

• selftune added to CONVERGED_FEATURE_KEYS + FEATURE_GLOBAL_FLAG
• Real call site(s) migrated
• Tests mirroring the existing rag/reputation/safety/unifiedComment feature-activation test pattern
• A check against the shadow-soak mechanism
• Docs updated

Expected outcome

An operator can turn selftune on/off per-repo purely through config, using the same mechanism as the other converged features.

Effort

S — mechanical migration onto an existing, proven mechanism.

[linear-code]

JSO-3154