v0.4.0: Execution hardening, cache locking, and host prompt bindings

Release focused on execution-start hardening, repository-cache synchronization safety, and prompt binding expansion.

Added

• repository-cache synchronization locking with bounded wait, stale-lock recovery, and recreation coverage for pull refresh failures;
• support for prompt-variable bindings from same-event host-provided attributes under host.*, alongside event.* and workspace.*;
• integration coverage for accepted webhook execution, host-backed prompt bindings, and rejection of agent profiles without promptRef;
• unit coverage for repository-cache locking, runtime lock configuration, host-backed prompt binding resolution, and enriched webhook failure paths.

Changed

• runtime execution contracts now consistently assume Docker as the only supported execution boundary;
• agent execution is rejected before preparation side effects when the selected profile does not expose execution.handler.promptRef;
• webhook failure handling now records execution failures explicitly in the event-scoped processing log;
• published README.md, docs/, and ai/ documentation now align on host.* prompt bindings, prompt materialization rules, and provider responsibilities;
• development dependencies and lockfile state were refreshed after npm update, including npm-sourced @teqfw/di and newer @types/node / undici-types entries.