feat: pidfile-based socket discovery (#47, phase 1)

[Cannon07]

Phase 1 of #47 — replaces OS-specific socket-glob discovery with a self-registered pidfile that each nvim writes on setup().

Summary

• Each nvim running code-preview.setup() writes ~/.local/state/code-preview/sockets/<pid> containing its RPC socket path and current cwd (two lines).
• bin/nvim-socket.sh checks this directory first, validates the socket is responsive, applies the same match-or-parent-cwd rule as before, and falls back to the existing /var/folders / /tmp / $XDG_RUNTIME_DIR discovery if no pidfile matches.
• :CodePreviewStatus and :checkhealth code-preview both surface pidfile state.

Why

The glob-based discovery relies on lsof, compgen, kill -0, and /proc — none of which exist natively on Windows. This is the main blocker for #46. The pidfile is computable from any platform and stores cwd directly, eliminating the lsof lookup that today returns the process's startup cwd rather than the user's current :cd.

Behavior preserved

• Multi-instance routing via the same match-or-parent-cwd rule
• NVIM_LISTEN_ADDRESS override (still checked first)
• Single-instance / fallback case (legacy globs still run when no pidfile matches, so users running an older nvim alongside the upgraded plugin keep working)

Behavior improved

• Liveness check is now socket-connect, not kill -0 — catches hung nvim with a live PID
• cwd is read live from nvim and refreshed on DirChanged, so :cd-ing to a different project routes correctly without restarting nvim

Scope

This is phase 1 of a four-phase migration. The pidfile itself is permanent — phases 2–4 (orchestration porting to Lua) will continue to read from the same pidfile dir. nvim-socket.sh will be removed entirely in a later phase.

Out of scope here:

• Windows-portability of the bash itself (-S test, path separators) — not worth it on a file we're deleting
• Phase 2+ orchestration changes

Test plan

• Pidfile written with correct socket + cwd on setup()
• DirChanged refreshes cwd line on :cd
• VimLeavePre removes the pidfile on :qa
• nvim-socket.sh resolves the right socket via pidfile (single instance)
• Multi-instance routing — two nvims at different cwds, each gets its own edits
• Subdirectory match — cd repo/lua still resolves to nvim opened at repo/
• Fallback to legacy globs when pidfile dir is empty/missing
• End-to-end Claude Code edit lands in the correct nvim
• :CodePreviewStatus shows pidfile path
• :checkhealth code-preview shows pidfile registration status

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