Remove legacy task input aliases

[chubes4]

Summary

• Delete unsupported legacy compatibility modules for task input, recipe alias, sandbox tool metadata, and PHP result/input normalization.
• Require canonical task and recipe contracts: goal, agent_bundles, sandbox_tool_policy, inputs.extra_plugins, steps-json, and canonical sandbox runtime metadata.
• Update docs, recipes, fixtures, and smokes to use canonical inputs and reject legacy aliases instead of normalizing them.

Fixes #766.

Deleted legacy paths

• Removed packages/runtime-core/src/legacy-task-input.ts, recipe-compat.ts, and sandbox-tool-metadata-compat.ts.
• Removed packages/wordpress-plugin/src/class-wp-codebox-legacy-input-normalizer.php and class-wp-codebox-legacy-result-normalizer.php.
• Removed support for task, agentBundles, sandboxToolPolicy, inputs.extraPlugins, browser actions-json, legacy sandbox execution_location / transport_visibility metadata, and the homeboy/wp-codebox-task-request/v1 parent request schema.
• Removed Agents API legacy schema advertisements including legacy_abilities and x-wp-codebox-accepted-schemas.

Checks

• npm run build
• npm run task-input-contract-smoke
• npm run sandbox-tool-policy-smoke
• npm run command-registry-smoke
• npm run recipe-dry-run-smoke
• npm run runtime-action-adapter-smoke
• npm run wordpress-plugin-smoke
• npm run wordpress-recipe-builders-smoke
• npm run recipe-build-cli-smoke
• npm run agent-task-run-runtime-components-smoke
• npm run recipe-run-php-plugin-load-smoke
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the canonical-only compatibility removal, updated tests/docs, and ran local verification; Chris remains responsible for review and merge.

[chubes4]

Holding this PR before merge. The current shape mostly preserves and centralizes legacy compatibility paths, which leaves explicit technical debt unless each accepted legacy surface is tied to a concrete shipped-input or persisted-data requirement. Please revise toward removing unsupported legacy inputs by default, or document the exact required compatibility surface and keep only that bounded normalization.