platform: close Windows shell/Cursor and process-detection gaps

[subinium]

Problem

Several advertised macOS/Linux/Windows paths have platform-specific gaps.

Cursor paths on Windows

Cursor project directory names are decoded by splitting on dashes and rebuilding from POSIX root /, so drive-letter/UNC paths cannot be recovered (src/scanner/cursor_agent.rs:328-333). Existing fixtures are Unix-only.

PowerShell setup/profile selection

agf setup relies on $SHELL; PowerShell on Unix can inherit /bin/zsh and modify .zshrc. On Windows it guesses PS5 vs PS7 from directory existence, and success messaging prints the invalid PowerShell command source ... (src/shell.rs:129-233).

Bash profile regression

When .bashrc is absent, setup creates .bash_profile even on Linux. Bash then stops reading an existing .profile, potentially hiding PATH/environment setup (src/shell.rs:139-147).

Process detection and executable detection

Watch depends on pgrep and installation detection accepts any matching directory entry in PATH without checking regular-file/executable permissions (src/watch.rs:184-205, src/config.rs:99-168).

Acceptance criteria

• Add Windows drive/UNC Cursor fixtures and platform-correct path decoding.
• Support explicit agf setup --shell ...; for PowerShell use the current host's $PROFILE.CurrentUserAllHosts and correct reload guidance.
• Preserve existing Bash startup-file precedence and .profile behavior.
• Add a Windows/process backend or expose running status as unknown when unsupported; do not retry a missing tool every tick.
• Verify regular executable files and Unix execute bits when detecting agents.
• Run platform fixtures in CI.

[subinium]

v0.15.0 integration update

Merged in #91 (7a426265b49cd9757cb111902abc8fdfa34536e9). The exact merged
commit passed all nine CI jobs, including native Windows Rust/CLI tests and
macOS/Linux PTY acceptance. Tag publication is tracked separately by the
release workflow; this comment does not claim publication completed.

Covered in this release:

• Current/legacy Cursor paths, drive/UNC/foreign-path classification and bounded
  path recovery, with platform fixtures. Generic agent is not guessed as
  Cursor; AGF_CURSOR_CLI explicitly selects another executable.
• Executable discovery requires executable files and preserves the absolute
  first PATH match across a subsequent cwd change, including PATHEXT behavior.
• Unsupported process inspection is reported as unknown rather than idle;
  missing process tools are not repeatedly re-probed.
• Scoped PowerShell environment restoration and dedicated-child native exit
  codes are tested, including actual PowerShell processes on Windows CI.

Keeping this issue open: the remaining setup/profile-host selection and Bash
startup-file requirements need their own acceptance review. Windows Rust/CLI
fixtures do not prove a physical Windows console PTY or every provider's live
runtime behavior.