feat(logging): structured logging via tracing across all binaries

[jakob1379]

Replaces env_logger with tracing + tracing-subscriber behind a shared logging::init(), so the warnings the desktop UI already displays also reach the terminal.

The bug

emit_activity() only invoked the UI callback, and the CLI's bare env_logger::init() defaulted to error, hiding every existing log:: call site.

Activity now routes through emit_activity_with(), where the callback is a parameter rather than ambient state. Task-locals don't cross tokio::spawn, so the stream collectors captured the callback up front and had diverged from the logging path — in the desktop app, only the two pre-spawn "Launching…" events ever logged.

Configuration

• RUST_LOG sets the filter (default info). Empty or unparseable falls back to info rather than silencing the process.
• DIFFCORE_LOG_FORMAT=json switches to line-delimited JSON.
• Output prefers stderr whenever it is a terminal. The desktop app falls back to ~/.diffcore/desktop.log only when it is not — GUI bundles discard stderr (windows_subsystem = "windows" in release; Finder/.desktop launches drop it). DIFFCORE_LOG_FILE overrides either way.
• ANSI only on a real terminal, honouring NO_COLOR.
• Targets: activity (backend events), ir_cache (replacing the bespoke DIFFCORE_CACHE_DEBUG gate), refinement, command.

RUST_LOG=info,activity=warn diffcore analyze --base main   # mute per-event chatter
DIFFCORE_LOG_FORMAT=json diffcore analyze --base main 2> analyze.log.jsonl

Channels that showed the user something but logged nothing

JobHandle::fail, CommandError's IPC serialization (the funnel ~30 tauri commands report failure through), and codex error items — which logged at info as "Completed error" with no detail, and now carry the nested error message at error.

Two pre-existing redact_api_keys bugs

Both became reachable by routing activity messages through redaction:

• It byte-sliced at 500 bytes while activity messages cap at 180 chars. 183 CJK chars is 549 bytes, so it panicked mid-codepoint. The panic lands inside a tokio::spawned collector, surfacing as a misleading "Failed to join claude stdout task" and discarding the entire buffered LLM response.

• sk- matched inside ordinary words, and break then skipped real keys later in the same line:

  "Codex is running task-manager build"  -> "Codex is running ta[REDACTED_API_KEY] build"
  "disk-usage high; using sk-abcdef..."  -> unchanged   (real key NOT redacted)
  

Both fixed at the root with regression tests. Note update.payload remains unredacted — it does not reach the logs, only the UI and SSE stream.

Notes

• tracing-subscriber sits behind a default-off logging feature, so library consumers of diffcore-core don't link the subscriber stack (cargo tree -p diffcore-core -e normal shows zero matches).
• Existing log:: call sites and dependency logs bridge in via tracing-log. No call site was rewritten.
• Logs go to stderr, so diffcore analyze | jq still works.

Testing

1706 lib tests plus all integration suites and the tauri web suite pass; nix build is green. The regression test drives the real collect_claude_stream with a callback installed and was verified to fail (empty log output) against the pre-fix code.

Known follow-ups, deliberately out of scope

• codex_cli.rs drops non-JSON lines, so codex raw stderr produces no activity where claude's produces a WARN. Pre-existing.
• ~/.diffcore/desktop.log has no rotation once it is used.

Clippy

Measured against the merge-base with a matching toolchain:

	errors	warnings
baseline	10	107
this branch	7	107

No new findings. The three removed errors are the eprintln! calls in pipeline.rs that violated the crate's own #![deny(clippy::print_stderr)]. CI runs only cargo test --workspace --locked, so those deny attributes were never enforced — adding a clippy job would need the 7 remaining pre-existing errors addressed first, so I have left that out of this PR.