GA blocker: remove all deprecated commands, compatibility code, and legacy documentation

[pacphi]

Goal

Before the 4.0 GA release, remove every deprecated command, compatibility alias, legacy function, and superseded implementation approach. Each supported capability must have one stable, documented forward path; no deprecated runtime surface may ship in GA.

GA policy

• ak run is the sole stable execution surface. It is host-neutral and is the only path for explicit OpenCode execution.
• AQE remains provider-axis only. OpenCode is never an AQE provider, primary host, or vendor-diversity fact.
• A deprecated alias, wrapper, compatibility export, persisted compatibility shape, or legacy approach must be either migrated to a stable replacement or removed. Keeping it because it is convenient is out of scope for GA.

Required inventory and disposition

• Produce a complete, reviewed inventory of active deprecated commands, aliases, compatibility exports, legacy functions, persisted legacy fields, generated templates, tests, and user documentation. Classify each as migrate, remove, or deliberately retained historical record.
• Remove ak dual and all supporting dispatch, command, adapter, temporary-config, escalation-wrapper, test, and documentation paths after required behavior is migrated into ak run or explicitly retired.
• Remove ak provider and ak x provider aliases after their stable ak host equivalents and migration coverage are complete.
• Migrate every retained public behavior to one stable command/surface before deletion. This includes templates, route overrides, concurrency, timeouts, JSON output, escalation semantics, and error contracts; do not silently drop behavior.
• Replace providers.dualRouting and any other persisted compatibility name with a GA-appropriate canonical envelope. Provide deterministic one-way migration that preserves user routes, models, escalation intent, and provenance.
• Remove dead compatibility projections, helper functions, source modules, package dependencies, configuration writers, and test fixtures after consumers migrate.
• Remove all active user-facing documentation, CLI help, generated guidance-template, and example references to deprecated commands or legacy approaches. Historical ADR/archive material may retain context only when clearly marked historical and excluded from active guidance.
• Update upgrading/release guidance with one concise GA migration path, then remove it from ordinary operational documentation.

Verification and acceptance criteria

• A clean GA install exposes only stable commands and no deprecation warnings. Removed commands fail as unknown commands.
• Repository-wide checks verify no active source imports, dispatches, or executes a removed compatibility path.
• A documentation/help guard fails if active user documentation, generated guidance, or CLI help reintroduces a removed command or legacy approach.
• Migrated configuration preserves user-pinned routes, models, escalation intent, and provenance without duplicating old and new state.
• ak run owns the migrated execution behavior and retains OpenCode safety boundaries.
• pnpm run check passes, including migration, negative-command, and documentation-guard coverage.
• GA release notes document the breaking changes and stable replacements.

Dependencies

This is a pre-GA milestone task. It follows the OpenCode routability work in #76. The cleanup must preserve the established safety boundary: OpenCode is opt-in and supervised through ak run, while AQE remains provider-only.