README agent-usage docs

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Part of GOAL.md workstream C. Final GOAL.md criterion: README explains JSON mode, state root configuration, watch bounds, and the FileState dogfood flow.

Acceptance criteria

• Documents --json shapes, APS_HOME/--state-dir precedence, watch --count/--timeout/--jsonl
• Copy-pasteable agent examples that work from a clean checkout
• Links GOAL.md and the specs/ contracts

[0xLeif]

Shipped in PR #2 (aps 0.2.0, merge commit 62198af). Verified in review: gates green, behavior confirmed locally.