Add --log level and keep diagnostics off stdout

[jenstopp]

The client used to print diagnostic logs and errors to stdout, mixing them into command output and breaking the json/yaml formats. That is fixed, and how much the client reports is now configurable.

• New --log flag sets diagnostic verbosity: debug, warn (default), or none.
• Diagnostics and errors now go to stderr; stdout carries only command output, so piping and -f json|yaml stay clean.
• At the default warn level, warnings are collected into a short summary at the end of the run rather than interrupting output.
• Terminal output is color-coded (errors, warnings, file transfers, status); color turns off automatically when output is piped or redirected, or with -no-color.
• Ctrl-C now stops a running command cleanly.

The -v flag is unchanged.

[Copilot]

Pull request overview

This PR separates client diagnostics from command output so machine-readable formats (JSON/YAML) and piping remain clean, while adding configurable diagnostic verbosity and improving human-facing terminal output (color/markers) plus graceful Ctrl-C handling.

Changes:

• Introduces a --log flag and a custom slog handler that writes diagnostics to stderr and (by default) summarizes warnings at the end of the run.
• Updates formatters and RPC flow to keep stdout reserved for command output, with structured file-transfer and error/status rendering.
• Adds shared style constants (ANSI colors + markers) and TTY detection to gate color automatically.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/style/style.go	Adds shared ANSI + marker vocabulary and a single Colorize helper.
internal/output/text.go	Adds colorized markers for context/success/errors and introduces file-transfer rendering + humanBytes.
internal/output/text_test.go	Updates expectations for markers/colors and adds tests for error + file-transfer output.
internal/output/output.go	Adds TypeFileTransfer and FileTransfer payload type.
internal/output/oneline.go	Adds FileTransfer formatting for oneline output.
internal/output/json.go	Replaces log.Printf with slog.Warn for marshal failures (stderr diagnostics).
internal/output/yaml.go	Replaces log.Printf with slog.Warn for marshal failures (stderr diagnostics).
CONTRIBUTING.md	Documents stdout vs stderr rules and the new logging model/constraints.
cmds/dutctl/term.go	Adds TTY detection helper to gate color output.
cmds/dutctl/term_test.go	Tests TTY detection behavior for non-file writers and regular files.
cmds/dutctl/rpc.go	Uses slog, emits file-transfer events via formatter, and handles Ctrl-C via signal-cancelled context.
cmds/dutctl/dutctl.go	Adds --log flag, installs default slog handler, gates formatter color on TTY, and flushes diagnostics on exit.
cmds/dutctl/clilog.go	Implements --log parsing and a CLI-oriented slog.Handler with warn buffering + flush.
cmds/dutctl/clilog_test.go	Tests log mode parsing and handler behavior across warn/debug/none.
.golangci.yml	Enforces no direct stdout printing and restricts allowed slog entry points.

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