feat: [Phase 0] Dispatcher abstraction layer for Python bridge migration

[anandgupta42]

Phase 0: Dispatcher Abstraction Layer

Parent: #214

What

Create a Dispatcher module that wraps Bridge.call() with a native handler registry. Update all 77 tool files to use the dispatcher. Zero behavior change — everything falls back to the Python bridge.

Files to create

• packages/opencode/src/altimate/native/dispatcher.ts — routing layer with typed registry
• packages/opencode/src/altimate/native/index.ts — barrel exports

Files to modify

• All 77 tool files in packages/opencode/src/altimate/tools/*.ts — Bridge.call() → Dispatcher.call()

Feature flags

• ALTIMATE_NATIVE_ONLY=1 — throws if no native handler (CI gate)
• ALTIMATE_SHADOW_MODE=1 — runs both paths, logs mismatches

Acceptance criteria

• Dispatcher created with typed register() and call() functions
• All 77 tool files updated to use Dispatcher.call()
• bun test passes with zero regressions (bridge fallback active)
• ALTIMATE_NATIVE_ONLY=1 throws for all methods (none migrated yet)
• No Python behavior changes

[anandgupta42]

Completed in PR #221.