fix(dev-mcp): pin UTF-8 for PowerShell shell commands

[PresentJay]

Windows PowerShell 5.1 encodes what it writes to a native process with the legacy console codepage and substitutes ? for every character that codepage cannot represent. Prose piped into buzz messages send --content - loses its em dashes, curly quotes, and non-Latin text before the event is built and signed — and the command still exits 0, so the corruption is silent and permanent in message history.

buzz-dev-mcp spawns that shell for every dev__shell call, which is the path managed agents use to reach the CLI. This pins BOM-less UTF-8 on the session before the caller's command runs:

$OutputEncoding = [Console]::OutputEncoding = [Text.UTF8Encoding]::new(); <command>

That is the workaround from the issue, moved from the caller into the spawn so it holds for every session. A prompt-level instruction cannot: each channel is an independent, long-running agent session, so one can comply while another keeps using the legacy encoding.

Dispatch is on the resolved shell (powershell / pwsh), not the host OS, matching how shell_flag right above it already picks the command flag. Every other shell is handed the command unchanged.

Fixes #6527

Test

Two cases in crates/buzz-dev-mcp/src/shell.rs, over the command string that is actually handed to the shell:

• powershell_command_pins_utf8_before_the_callers_command — asserts the preamble comes first and the caller's command (em dash — café 測試 🐝) survives intact, for powershell.exe, pwsh.exe, and pwsh. Before the change it fails with the command returned bare.
• other_shells_receive_the_command_unchanged — bash, /bin/zsh, sh, and cmd.exe are untouched.

cargo test -p buzz-dev-mcp   101 passed / 0 failed
just fmt-check               clean
just clippy                  clean
just test-unit               9/9 lanes pass

Scope note: this covers how the command is constructed. The end-to-end Windows assertion the issue asks for — sending through a live harness and reading the accepted event's code points back — is not included here; it needs a Windows runner this change cannot provide.

[github-actions]

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