Add task execution target plumbing

[chubes4]

Summary

• Add generic task execution contracts for agents-api/task-input/v1, placement hints, executor target discovery, and task result envelopes.
• Register agents/run-task, agents/list-execution-targets, agents/get-task-run, and agents/cancel-task-run with filter-based executor discovery/dispatch.
• Add task run-control storage/helpers, smoke coverage, and docs clarifying executor providers own concrete side effects.

Testing

• php tests/task-execution-smoke.php
• composer smoke
• composer phpstan

Closes #309.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the generic task execution plumbing, smoke tests, and documentation; Chris remains responsible for review and validation.