feat: improve stdio handling with async writers

[branchseer]

Summary

Extends the existing "stdin suggestion" system to a full "stdio" system. Previously, only stdin was conditionally inherited vs null. Now, all three stdio FDs (stdin, stdout, stderr) are conditionally inherited vs piped, and the reporter provides Box<dyn AsyncWrite + Unpin> streams for receiving child process output.

Changes

New types and API

• StdioSuggestion replaces StdinSuggestion — two modes: Piped (stdin=null, stdout/stderr piped into AsyncWrite streams) and Inherited (all three inherited from parent)
• StdioConfig returned by LeafExecutionReporter::start() — contains the suggestion plus stdout_writer and stderr_writer async streams
• Removed stdin_suggestion() and output(kind, content) from LeafExecutionReporter trait
• Removed OutputKind from event.rs (only spawn.rs keeps its own OutputKind)

Reporter changes

• Removed the W generic from all reporters (PlainReporter, LabeledReporterBuilder, LabeledGraphReporter, LabeledLeafReporter, SharedReporterState)
• Display output (command lines, summaries, errors) written directly to std::io::stdout() using sync writes
• tokio::io::stdout() / tokio::io::stderr() provided as the AsyncWrite streams in StdioConfig
• PlainReporter returns tokio::io::sink() writers when silent_if_cache_hit && is_cache_hit

Execution flow

• spawn_with_tracking() — stdin is always Stdio::null(), takes &mut dyn AsyncWrite for stdout/stderr instead of an on_output callback
• New spawn_inherited() — inherits all three stdio FDs, no fspy tracking, just cmd.spawn() + wait()
• Decision matrix: inherited mode only when suggestion is Inherited AND cache_metadata.is_none() (caching fully disabled)
• Cache replay writes directly to StdioConfig writers
• In-process execution writes to stdout_writer from StdioConfig

Test updates

• Updated 8 stdio suggestion unit tests to use start() + assert StdioConfig.suggestion
• Updated task-list E2E "vp run in script" test: inner vp run now sees a terminal (inherited stdout) and shows interactive selector — test uses expect-milestone + Enter to select and run a task

[branchseer]

• feat: improve stdio handling with async writers #160 👈 (View in Graphite)
• main

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