agent: engine slop residuals B — eprintln → tracing sweep

Replaces 22 production eprintln! sites with structured tracing events
across 8 files. Closes Area B of the post-prognosis residuals plan
(docs/superpowers/plans/2026-05-12-engine-slop-residuals.md).

Files touched (22 sites):
- crates/hotkey/src/linux.rs (2) — hotplug watcher degraded-mode warnings
- crates/ai-formatting/src/pipeline.rs (1) — LLM cleanup fallback warning
- src-tauri/src/commands/transcription.rs (1) — chunking dispatch info
- src-tauri/src/commands/diagnostics.rs (1) — crashes-dir setup warning
- src-tauri/src/commands/tasks.rs (1) — malformed feedback row warning
- src-tauri/src/commands/power.rs (3) — App Nap acquire/release/fail
- src-tauri/src/commands/models.rs (5) — Whisper warmup lifecycle
- src-tauri/src/commands/live.rs (8) — session start, chunk dispatch,
  per-chunk delivery, inference errors, worker disconnects, listener
  loss, status-channel cascade

Levels: error for unrecoverable failures (inference disconnect, panic,
status cascade), warn for recoverable degradation (LLM fallback,
malformed rows, App Nap fail, hotplug watcher fail), info for lifecycle
(session start, chunk processed, App Nap acquire/release, warmup
complete, chunking dispatch), debug for per-chunk noise (speech-gate
skip, chunk dispatch).

Two new dependencies and two new filter targets:
- tracing = "0.1" added to crates/hotkey and crates/ai-formatting
- Default EnvFilter in src-tauri/src/lib.rs::init_tracing extended with
  magnotia_hotkey=info,magnotia_ai_formatting=info so the new targets
  emit at the default level

Out of scope (intentional, left as-is):
- crates/mcp/src/main.rs — CLI binary, stderr is the log contract
  (module docstring) so the JSON-RPC stdout stream stays clean
- crates/*/tests/*.rs and crates/core/examples/tuning_log_demo.rs —
  test/example diagnostic output relies on --nocapture stdio semantics

Discovery during sweep (not fixed — separate follow-up): hotkey crate
has 6 existing log:: calls (log::error/warn/info/debug) but the
workspace builds tracing-subscriber without the tracing-log feature, so
those events are currently silent. Worth a follow-up to either add the
tracing-log bridge or migrate hotkey's existing log:: calls to
tracing::.

Verification:
- cargo fmt --all
- cargo check --workspace --all-targets — clean
- cargo test --workspace — 330+ tests, zero failures
- rg eprintln! src-tauri/src/commands/ crates/hotkey/src/ crates/ai-formatting/src/ — zero hits

Pre-existing working-tree churn in crates/llm/, src/lib/pages/,
src/lib/utils/saveMarkdown.ts and the untracked phase10a dogfood notes
deliberately left unstaged per Jake's instruction.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/ai-formatting/Cargo.toml

@@ -8,3 +8,4 @@ description = "Text post-processing pipeline: filler removal, British English co
 magnotia-core = { path = "../core" }
 magnotia-llm = { path = "../llm" }
 regex-lite = "0.1"
+tracing = "0.1"

crates/ai-formatting/src/pipeline.rs

@@ -91,8 +91,9 @@ pub fn post_process_segments(
                         replace_segments_with_cleaned(segments, cleaned.trim());
                     }
                     Ok(_) => {}
-                    Err(err) => eprintln!(
-                        "[ai-formatting] LLM cleanup failed, keeping rule-based output: {err}"
+                    Err(err) => tracing::warn!(
+                        error = %err,
+                        "LLM cleanup failed, keeping rule-based output"
                     ),
                 }
             }