Add safe execution workspace primitive

[chubes4]

Summary

• Adds an opt-in Agents API safe execution workspace target for host-approved, site-isolated filesystem workspaces.
• Registers scoped workspace prepare/list/read/write abilities only when the feature is enabled and a valid isolated root is configured.
• Adds pure-PHP smoke coverage for default-off behavior, target/ability registration, contained file access, traversal rejection, and site-root isolation rejection.

Verification

• homeboy lint --changed-only
• php tests/safe-execution-workspace-smoke.php
• php tests/task-execution-smoke.php
• git diff --check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the safe workspace primitive, smoke coverage, and verification workflow; Chris remains responsible for review and final acceptance.