* fix(sh): escape `/C` so MSYS doesn't drop the cmd switch

The sh shim generated for a `.cmd` / `.bat` target runs the script via
`exec cmd /C "<path>" "$@"`. Under Git Bash (MSYS), the path-conversion
layer that runs when bash launches a native Win32 process rewrites
arguments matching POSIX-path heuristics — a bare `/C` is treated as a
path and rewritten to `C:\`. cmd.exe then never sees the `/C` flag,
starts interactively, and reads the rest of the calling script as input
until it hits EOF.

Prefixing with `//` is the MSYS escape: `//C` survives the translation
and reaches cmd.exe as `/C`. The cmd shim is unaffected (`%*` argument
passing in cmd.exe doesn't get this treatment), so the change is scoped
to `generateShShim`.

Bug introduced in #46; manifests as cmd.exe banner output when invoking
any cmd-shim-wrapped `.cmd` from Git Bash.

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Written by an agent (Claude Code, claude-opus-4-7).

* test(e2e): regression test for MSYS /C escape

Generates an sh shim wrapping a `.cmd` target and invokes it via Git
Bash. Asserts the script's output appears (proving the `/C` switch
reached cmd.exe) and that cmd.exe's interactive banner did not (proving
the bug — where MSYS rewrote `/C` to `C:\` — is not present).

Windows-only; the suite is skipped elsewhere.

---
Written by an agent (Claude Code, claude-opus-4-7).

* fix(sh): scope MSYS escape to the cmd runtime

Apply the /C → //C rewrite only when opts.prog is 'cmd' (the runtime
inferred for .cmd/.bat targets). Avoids mangling legitimate /C-like args
from shebang-derived configurations on non-MSYS systems, where the
program would see //C verbatim.

Addresses review feedback on #55.