feat(cli): global -q/--quiet flag to suppress the WARN log preamble (REQ-151, #353)

[avrabe]

Closes the --quiet half of #353's agent-ergonomics asks.

Problem

Every rivet invocation can emit WARN-level log lines before a command's real output (e.g. [WARN rivet] could not load externals: …), which is noise for scripted/agent consumption — especially with --format json. There was -v/--verbose to raise the log level but no way to lower it below the default warn.

Change

A global -q/--quiet (mutually exclusive with --verbose) sets the log filter to error: the WARN preamble is suppressed, while the command's own stdout and hard-error reporting stay intact. Pairs with --format json.

$ rivet list                 # [WARN rivet] could not load externals: …  ⏎  REQ-1 …
$ rivet --quiet list         # (no stderr)                                ⏎  REQ-1 …
$ rivet -q list --format json   # clean JSON, no preamble

Verification

• New cli_commands::quiet_suppresses_warn_preamble: on a project with a misconfigured external, default stderr contains WARN, --quiet stderr does not, and stdout still lists the artifact. (Hard errors verified still shown under --quiet.)
• clippy --all-targets + fmt clean; rivet validate PASS (191), rivet docs check PASS.

Also (triage, verified — no code needed)

#353 part 3 ("tight modify loop silently no-op'd"; "is modify's exit code reliable / can rapid successive modifys race?") does not reproduce on current main: 90/90 rapid sequential rivet modify calls (3 trials × 30) all exited 0 and persisted; validate PASS. Likely resolved by intervening modify rework, or was environment-specific to the reporter's 0.13.3 — and it's superseded by modify --where (#380) regardless. Commenting this on #353.

Implements: REQ-151

🤖 Generated with Claude Code

[github-actions]

📐 Rivet artifact delta

Change	Count
Added	1
Removed	0
Modified	0
Downstream impacted (depth ≤ 5)	0

Graph

graph LR
  REQ_151["REQ-151"]:::added
  classDef added fill:#d4edda,stroke:#28a745,color:#155724
  classDef removed fill:#f8d7da,stroke:#dc3545,color:#721c24
  classDef modified fill:#fff3cd,stroke:#ffc107,color:#856404
  classDef overflow fill:#e2e3e5,stroke:#6c757d,color:#495057,stroke-dasharray: 3 3

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Added

• REQ-151

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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