fix(cli): keep stdout valid JSON under --json (#546)

[tonylampada]

Description

Fixes #546. Agents piping roboflow --json into jq break on search --export, image upload <dir>, model upload and version download, because SDK status lines land on stdout before the JSON. Under --json these SDK calls now print to stderr, so stdout is only the JSON document; text mode is unchanged. Serves objective O8 — one canonical API shape on every surface (CLI).

Note: #547 (external) takes the same approach; this one adds HTTP-level tests with responses. Merge whichever the maintainers prefer.

What changed

• New sdk_output_to_stderr(args) in _output.py: redirects stdout→stderr only under --json.
• The four handlers wrap their printing SDK call in it. stderr, not suppress_sdk_output(), so per-image [ERR] lines (Failed uploads are reported as success: Version.deploy() / Workspace.deploy_model() don't raise, and roboflow model upload / roboflow image upload <dir> exit 0 #545) stay visible.
• model upload now silences workspace/project init like the other handlers.
• Separate commit: datetime → dt alias in 2 files; the latest (unpinned) ruff flags ICN001 and fails CI on every PR.

How has this change been tested?

• New automated tests added: tests/cli/test_json_stdout.py, one per command, HTTP mocked with responses, json.loads(stdout); plus a text-mode regression test. The 4 --json tests fail on main.
• Full suite: 1234 OK. Evidence: https://gist.github.com/tonylampada/6b0d371853dbf289d23a2555746a519e; no staging needed (CLI output routing only).

Any specific deployment considerations

None.

Next step

Done for #546; a general "redirect stdout for the whole command under --json" would cover future handlers (deliberately out of scope).

PLAN
touches   CLI --json on search --export, image upload <dir>, model upload, version download → SDK prints move stdout→stderr; text mode unchanged
touches   model upload → workspace/project init now silenced (drops "loading Roboflow workspace...")
tests     each of 4 commands under --json: stdout parses with json.loads, SDK lines land on stderr · new tests/cli/test_json_stdout.py (HTTP mocked w/ responses, CliRunner)
tests     text mode keeps SDK lines on stdout · same file
review    CLAUDE.md "Agent experience: --json stable schema" — stdout must be only the JSON doc
review    CLAUDE.md "wrap SDK calls in suppress_sdk_output" — deliberate exception: per-image [ERR] lines (#545) stay visible on stderr
verify    python -m unittest tests.cli.test_json_stdout -v → 5 OK; fail on main for the 4 --json tests
verify    make check_code_quality → clean

🤖 Generated with Claude Code

[tonylampada]

Deliver checks (SLM-101)

• Own review + verify recipe: python -m unittest tests.cli.test_json_stdout 5 OK; 4 --json tests fail on main; full suite 1234 OK; CI green
• Evidence gist: https://gist.github.com/tonylampada/6b0d371853dbf289d23a2555746a519e
• PR description per contract (<40 lines, objective O8, plan block)
• Review round: /jarbas-review (ruling: no bot on this repo) — 1 fixed, 3 answered "won't fix, out of scope" (see comment below)

[tonylampada]

Review round (jarbas-review), one pass:

• Fixed: CLAUDE.md / CONTRIBUTING.md command guide now names sdk_output_to_stderr(args), so new handlers don't reintroduce non-JSON stdout.
• model upload returns {"status": "uploaded"} / exit 0 when Workspace._upload_zip / Version.deploy print-and-swallow an HTTP error: won't fix, out of scope: pre-existing SDK error-handling bug, not stdout routing; needs the SDK to raise. Worth its own issue.
• Single-file image upload (project.single_upload) can still print to stdout (classname line, file-type mismatch): won't fix, out of scope: --json output is not valid JSON for search --export, image upload <dir>, model upload and version download #546 names the four commands; same helper applies in a follow-up.
• Whole-command stdout redirect under --json to cover future handlers: won't fix, out of scope: the per-handler variant is what --json output is not valid JSON for search --export, image upload <dir>, model upload and version download #546 proposes; the docs change above covers new handlers.

[tonylampada]

Night-loop drill run; closed by Selma. #547 is the external fix for #546.