fix(planning): interview round.sh CLI hygiene for --dir, JSON status, Python 3 and --open #8013

Refs: #7303

Agent Brief

Type: Bug
Summary: Re-verify and fix DX-4, DX-6, DX-8, DX-10 and DX-14 in the interview page's control script.

Current behavior:
Per #7303: --dir after the subcommand works on only some subcommands though help says it may (DX-4); status, lease, doctor and validate have no JSON output (DX-6); round.sh accepts any python that runs, including Python 2 (DX-8); --open fails silently when no browser resolves (DX-10); help has no examples and apply --help does not name the ops (DX-14).

Desired behavior:
Each item is checked on main first. Confirmed ones are fixed: --dir is accepted after every subcommand or help stops saying so; the four commands take --json with a documented shape while their text output is unchanged; round.sh rejects an interpreter below Python 3 with its missing-prerequisite message and exit 2; --open prints one stderr line when no browser opens; help carries examples and apply --help lists the op names. Unconfirmed items are listed as not reproduced in the PR body.

Key interfaces:

• round.sh's interpreter probe and round.py's argument parser and subcommand outputs.

Acceptance criteria:

• scripts/run-test-file.sh plugins/planning/surface/test_round.py passes with new cases for each fixed item.
• scripts/run-test-file.sh plugins/planning/surface/surface.test.sh passes.
• The PR body marks each of DX-4, DX-6, DX-8, DX-10 and DX-14 fixed or not reproduced.
• A planning changelog fragment is added and scripts/check-changelog-fragments.sh passes.

Out of scope:

• Exit-code changes (DX-3), the shared session-bridge files, and any interview SKILL.md text.
• Rebase over open PRs touching the surface (fix(planning): keep Q N register ids when syncing a gapped ledger #7913, fix(rendered-views): audit planning surface escapes and document two escape exceptions #7943, feat(planning): shared interview sessions #7670) if they merge first.

This was generated by AI during backlog shaping. Split from #7303; the owner approved filing it in an attended backlog-shaping session on 2026-10-18.