refactor(trace): unify in-process trace-trigger surface

[max-sixty]

Three small consolidations in the trace-trigger surface, following up on #2539.

What this changes

1. Cmd::stream() emits [wt-trace] cmd=... natively. Previously only Cmd::run() and Cmd::pipe_into() emitted per-subprocess records — stream() was a hole, which #2539 patched with a Span::new(\"execute_shell_command\") wrapper at the foreground hook/alias call site. The two surrogates aren't equivalent: spans render under cat: \"wt\", subprocess records under cat: \"git\"/\"network\", and spans don't carry the ok flag, so hook/alias child status was invisible in chrome traces. Stream now emits a record at every exit point (spawn fail, stdin write, wait fail, signal-derived exit, SIGPIPE-as-success, non-zero status, success) via a small WtTraceLog helper that mirrors ExternalCommandLog's shape.

2. Drops the Span::new(\"execute_shell_command\") workaround in commands/command_executor.rs. With Cmd::stream() emitting natively, the wrapper is redundant — foreground hook/alias step time is now captured by the canonical subprocess record (cat=git/network/none) instead of a generic span (cat=wt).

3. Re-exports trace::instant from trace::mod alongside Span. Deletes the shell_exec::trace_instant shim (a one-line re-export of trace::emit::instant) and migrates all 14 callers in commands/picker/mod.rs and commands/list/collect/mod.rs to worktrunk::trace::instant. Symmetric public API: trace::Span for scopes, trace::instant for milestones — neither lives under shell_exec anymore, since neither has anything to do with shell execution.

Verification

Smoke-tested with RUST_LOG=debug wt <alias>: cmd=\"echo hello-from-stream\" ok=true and cmd=\"exit 7\" ok=false both fire correctly. Span(\"execute_shell_command\") no longer appears in the trace. Full pre-merge hook (3430 tests + clippy + lints) passes locally.

[worktrunk-bot]

codecov/patch is failing — 49/55 hits, 6 misses on src/shell_exec.rs. The uncovered lines are the two new trace_log.errored(&e) calls in the rare error paths inside Cmd::stream():

• L1335 — stdin.write_all(content) failing with a non-BrokenPipe error
• L1405-1409 — child.wait() returning Err

Both paths existed before this PR (just returning errors via ? / e.into()); the new code adds a trace_log.errored(&e) call before the same return. They're hard to hit in unit tests — write_all failures other than BrokenPipe need the child to close stdin abnormally mid-write, and child.wait() failing is rarer still.

CLAUDE.md asks to investigate the gap rather than ignore it. Two options I see:

1. Drop the errored() calls in those two paths and rely on the trace gap being acceptable for paths that already return an error to the caller. The PR's primary motivation (closing the gap that Span::new("execute_shell_command") was working around) is the success and signal/exit paths, which are still fully covered.
2. Keep them and approve through the codecov failure — they're cheap defensive emissions and the failure modes are real (e.g., a kernel-killed child between spawn and wait).

I'd lean toward (1) for codecov compliance, but happy to push either direction. Dismissing my approval per CLAUDE.md's codecov rule — let me know which way you'd like and I can push the fix.

codecov/patch failed — 6 uncovered lines in shell_exec.rs error paths