refactor(cli): replace profiles with agent presets

[tumberger]

Summary

• make SandboxPolicy the only user-authored policy model across the Rust API and CLI
• replace the schema, inheritance, registry, and additive user-file machinery with four small CLI-local agent presets
• keep agent detection, default state access, sensitive-path protection, and known hook locations as typed product data
• move all agent knowledge out of sandy-core
• replace --profile and --profile-file with --agent and complete --policy-file authoring
• retain separate file and executable authority and add replacement live policy-file coverage

Stack

1. refactor(policy): separate launcher and subprocess capabilities #50 — separate launcher and subprocess capabilities
2. feat(cli): run with complete policy files #51 — complete CLI policy files
3. This PR — one policy authoring model and small built-in agent presets

Resulting boundary

• Rust callers: construct or parse one complete SandboxPolicy
• CLI policy authors: provide the same complete JSON document
• CLI defaults: select a small built-in agent preset
• core and native backends: receive only typed policy; they know nothing about agents

Verification

• make check (dependency audit completed separately because the sandbox cannot write Cargo's shared advisory lock)
• package-consumer, formula, and release-version checks
• strict x86_64-unknown-linux-gnu Clippy from macOS
• workspace unit, CLI, documentation, and ignored live-test compilation

This change deletes substantially more machinery than it adds. Draft until the complete stack is reviewed together.