fix(share): honor --json on arc share list

[bfirestone]

Summary

arc share list --json was silently emitting the tab-separated text output, ignoring the global --json flag. The handler in cmd/arc/share.go called fmt.Printf directly instead of branching on the package-level outputJSON, the way arc plan, arc label, and friends already do.

$ arc share list --json
e79y2k99    local   http://localhost:7432   docs/review_howto.md     ← ignored --json
fgqfxxhp    shared  http://localhost:7436   docs/plans/2026-04-29-shared-review.md

After this fix:

$ arc share list --json
[
  {
    "id": "e79y2k99",
    "kind": "local",
    "url": "http://localhost:7432",
    "key_b64url": "...",
    "plan_file": "docs/review_howto.md",
    "created_at": "2026-05-01T07:25:07Z"
  },
  ...
]

Why this matters

The marketplace plugin's /arc:plan skill now relies on arc share list --json | jq to look up the local file path for a given share ID (since arc share show returns the decrypted plan content, not metadata). Until this fix, that lookup silently failed.

Surface choices

• edit_token is omitted from JSON. It's a bearer secret gated behind the Author URL — it doesn't belong in machine-readable output that might get teed to logs or CI artifacts.
• key_b64url is included. This preserves parity with the text output, where the same key already travels inside the printed Share URL fragment (#k=<key>). Excluding it from --json would create inconsistent surface area without improving security posture.
• Empty keyring returns []. Not the human (no shares) string, so JSON consumers can [].length without parsing.

Test plan

• TestRunShareListJSON — seeds two shares, asserts JSON parses to a 2-element array containing both IDs, asserts edit_tokens don't leak
• TestRunShareListJSONEmpty — empty keyring returns valid []
• make build-quick + ./bin/arc share list --json against my local keyring → matches expected schema
• go test ./... — full suite green
• make lint — clean