Port the dev-server pane to the platform launch dialect (Windows)

[h0x91b]

The Dev Server button and dev3 dev-server start were dead on Windows: runDevServer called assertPosixLaunchDialect("the dev-server pane") as its first statement, so the native path never ran. Arseny reproduced it on his own box — issue #1387.

Two things were wrong, and fixing either alone is a trap:

1. The guard was in the wrong place. What needs tmux is the nested dev3-dev-<id> session plus the viewer pane that attaches to it — not the pane concept. The guard moved down to that branch (a Windows task is always native, so it is unreachable there).
2. The wrapper was hand-written bash (#!/bin/bash, set -x, [ $EXIT_CODE -ne 0 ], read -n 1 -s) launched through a hardcoded /bin/bash. Porting only the launch would have been worse than the outage — PowerShell half-runs bash text and looks like it started.

The body now lives in src/bun/dev-server-script.ts, authored in the launch dialect (traceOn/traceOff added for set -x / Set-PSDebug), and the pane launches it through generatedScriptLaunch/generatedScriptName.

Third defect, found on the way, in already-merged code: AUX_PANE_PURPOSES markers carried file extensions (dev.sh, col-agent.sh). The dialect names a generated script .ps1 on Windows, so a pane launched there was invisible to every later lookup — is it running, replace it, stop it. Extensions dropped; this also repairs the column-agent pane on Windows.

POSIX moved by exactly three lines, deliberately, pinned in their new form: the two echos became one printf, and read -n 1 -s became the dialect's shell-portable read (identical under bash, which launches the file). platform-launch-posix-golden.test.ts is untouched and green — no other wrapper moved.

What this does NOT fix, and cannot: a project's devScript is the user's own text in the user's own shell. bun run dev is valid in both dialects; VITE_PORT=\${DEV3_PORT0:-5173} bun run dev is a PowerShell parse error. What changes is that the failure is now the user's script failing in a live pane instead of dev3 refusing to open one. Documenting that caveat next to devScript is a separate, open product question.

Proof

Windows workflow run on this branch before merge: 31887058798 — package-runtime (windows-latest) green, both new steps green. From its log on the real runner:

← runDevServer done (native pane)          # the refusal is gone on the native path
error: the nested dev-server tmux session requires the tmux backend  # and still fires where it belongs
ok - the dev command actually executed (it wrote its record)
ok - the task env block reached the command (DEV3_TASK_SEQ=1546)
ok - the pane shows the real exit code                # crash reported, no hang on the keypress prompt
ok - the pane launches ...powershell.exe for ...-dev.ps1

Mutation branch mutation/dev3-dev-server-bash-hardcode restores the /bin/bash hardcode to confirm the Windows step goes red for it.

Fixes #1387

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🔗 Origin task in dev3: open in dev3 · dev3://task/723a9754-9241-4fab-a26b-6dd87f347f98