--json output for core commands

[0xLeif]

Part of GOAL.md workstream A.

Add a --json flag to the README core commands (get, set, dump, keys, reset) so agents get machine-readable output instead of parsing human text.

Acceptance criteria

• --json on each core command prints a single stable JSON document to stdout
• Shapes are documented (field names, types, error shape) and versioned deliberately
• Errors in JSON mode print a structured error object and exit non-zero
• Tests assert exact JSON for each command; smoke.sh covers at least get/set
• specs/aps-cli/aps-cli.spec.md updated in the same change (AGENTS.md rule)

[0xLeif]

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