feat(core): enable portable shell scanner on local and dev channels

[kitlangton]

Why

With #53277 (Bash/Zsh/Dash/POSIX) and #53404 (PowerShell) merged into v2, the portable shell scanner is ready to be dogfooded across all local source runs (channel === "local") and team v2 preview builds (channel === "dev").

What Changes

Defaults experimental.portable_shell_scanner to true when ctx.app.channel is "local" or "dev", while keeping "latest" (stable releases) opt-in and allowing explicit experimental.portable_shell_scanner: false to opt out on any channel:

const portable =
  Config.latest(yield* config.entries(), "experimental")?.portable_shell_scanner ??
  (ctx.app.channel === "local" || ctx.app.channel === "dev")

Scope

• packages/core/src/tool/plugin/shell.ts
• packages/core/test/tool-shell.test.ts

Verification

cd packages/core
bun run test test/tool-shell.test.ts
bun typecheck