fix(install): replace the flashduty already on PATH by default

[ysyneu]

Problem

Without FLASHDUTY_INSTALL_DIR, install.sh always installs to /usr/local/bin. If the flashduty the shell runs lives elsewhere (~/.local/bin from the installer's own non-interactive fallback, ~/go/bin from go install, …), an upgrade drops a second copy into /usr/local/bin that the earlier PATH entry keeps shadowing:

$ flashduty update
...
[flashduty] Installed to /usr/local/bin/flashduty
Update complete. Run 'flashduty version' to verify.
$ flashduty version
flashduty version 1.5.10 ...

flashduty update in v1.5.11+ passes the running binary's location, but releases up to v1.5.10 do not, and they fetch the latest install.sh from the CDN — so fixing the installer also repairs their next update. Re-running the documented curl … | sh hits the same problem on any version.

Change

• Default the install directory to that of the flashduty ($INSTALLED_NAME) found on PATH; fresh installs still use /usr/local/bin. Explicit FLASHDUTY_INSTALL_DIR is unchanged.
• After installing, warn when the shell still resolves a different copy (replaces the "not in PATH" check, which it subsumes). Directories are compared so dash's <dir>//flashduty output for trailing-slash PATH entries doesn't false-alarm.
• Refuse when FLASHDUTY_MANAGED_BY_RUNNER=1, matching flashduty update: flashduty-runner ships its own copy.
• README / README_zh default updated.

Tests

New cases in tests/install_sh_sudo_test.sh: replaces the copy on PATH (with a trailing-slash PATH entry), warns when a non-writable copy on PATH shadows the fallback install, refuses under the runner flag. All three fail against the previous install.sh and pass now.

• ubuntu:24.04 (/bin/sh = dash): shellcheck + full test script pass.
• macOS arm64, end to end against the real CDN: a v1.5.10 binary first on PATH, no FLASHDUTY_INSTALL_DIR → replaced in place, flashduty version reports 1.5.22, nothing written to /usr/local/bin or ~/.local/bin; with FLASHDUTY_MANAGED_BY_RUNNER=1 → exits 1 before downloading.

Rollout note

The install-scripts workflow mirrors install.sh to the CDN on merge to main, and the CDN caches it, so the cache for flashduty-cli/install.sh needs a refresh after merge. Windows install.ps1 is unchanged.