refactor(ui): route stdin probes and prompt reads through pkg_io

[indaco]

Description

Aligns the two remaining stdin reads in src/ui/ with the seeded pkg_io so AppCtx.io redirection observes them like every other I/O in the module:

• confirmTyped now reads via std.Io.File.readStreaming(pkg_io, ...), matching the isTty probe it already routes through pkg_io. Behind a small extracted helper so unit tests can drive the read against a pipe instead of needing a real TTY.
• The OSC 11 background probe in ui/color reads its response the same way, removing the last std.posix.read(stdin) bypass in the UI layer.

Also pins the quiet/verbose/debug/dry_run/mode/ndjson mutation contract in a comment: these flags are configured by argv parsing before any worker thread spawns, so plain reads/writes stay race-free; future runtime mutators must migrate the flag and its readers to @atomicLoad/@atomicStore (matching post_install_emit next door). Promoting them eagerly cost ~3 KB of code that crossed a Mach-O page boundary into a 16 KB on-disk hit for a race no caller actually produces - a real binary-size regression for a theoretical concern.

Related Issue

• None

Notes for Reviewers

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