feat: add public policy preflight checks for paths, commands, and URLs

[jy-tan]

Summary

Exposes Fence's policy evaluation as a public preflight API, so embedders can use one fence.json as the source of truth for operations that don't go through the sandbox (e.g. agent tools like glob/read_file/web_fetch built on the Go stdlib).

Resolves #189.

func CheckReadPath(cfg *Config, path, cwd string) error
func CheckWritePath(cfg *Config, path, cwd string) error
func CheckCommand(cfg *Config, command string) error
func CheckURL(cfg *Config, rawURL string) error

Thin exports of the internal predicates the hook integrations already use, so hook-mode, wrap-mode, and library checks can't drift. No Manager or proxies required. Denials are typed errors with matched rule and reason. These are policy preflights, not enforcement - the sandbox and proxies remain authoritative (documented, including the CheckURL declared-URL-only caveat).

Changes

• internal/sandbox/path_check.go: generalize PathWriteBlockedError -> PathBlockedError (adds Op; write messages unchanged); add CheckReadPath mirroring wrap-mode read semantics. Dangerous-path protection stays write-only by design.
• pkg/fence/fence.go: export the four Check* functions + typed-error aliases. CheckCommand is WrapCommand's preflight (chain/nested sh -c parsing, SSH policy) without a Manager.
• Tests for read semantics and all four checkers; "Policy Checks (Preflight)" section in docs/library.md.