cli: --dry-run flag for sb exec (evaluate policy, emit receipt, skip execve)

[tomjwxf]

Context

AGT's GovernanceSkill.check_policy(action, context) is pure evaluation —
it returns a PolicyDecision without actually running the action. sb exec
today evaluates and runs. The SbRuntimeSkill shim
currently works around this by reading the receipt the sb binary emits
before it calls execve, which is brittle.

A first-class --dry-run flag would:

• Evaluate Cedar
• Emit a signed receipt (marked `dry_run: true` in its context)
• Exit 0 for allow, 2 for deny
• Not apply sandbox
• Not call execve

Acceptance criteria

• `sb exec --dry-run --policy P -- CMD ARGS...` exits 0/2 based on policy
• Emitted receipt has `context.dry_run: true`
• `SbRuntimeSkill` updated to pass `--dry-run` and `SB_DRY_RUN` env
  handling removed
• Unit test + CI smoke for both allow + deny paths

Release target

v0.1.1.