fix: don't embed shell-unsafe install paths in statusline setup nudge (#224)

The SessionStart nudge built a statusLine command by interpolating the
plugin's __dirname path into a double-quoted shell string. A clone path
containing shell metacharacters (quotes, &, $, backtick, ;) could break
out when the suggested command later runs via the statusline shell.

Low severity in practice: the path is the install location, so triggering
it requires installing into a maliciously-named directory, i.e. the
attacker already controls the filesystem. Hardening it anyway.

Gate the snippet behind isShellSafe() (allowlist of ordinary path chars,
allowing : \ / for normal Windows and POSIX paths). Unsafe paths fall
back to a manual-setup instruction instead of an embeddable command. An
allowlist beats a per-shell escaper, which is its own edge-case bug farm.

Refs #200