docs(persist): add privacy note about plaintext session storage

Persistence writes file URIs, ranges, titles, provider names,
pin/buffer-mode flags, parent popup ids, and capture timestamps as JSON
under stdpath("state")/peekstack/, alongside per-session created_at and
updated_at metadata. Spell that out in the README and vimdoc so users
with persistence enabled can decide whether to enable it on shared
machines or sensitive repositories.

Author: mhiro2

README.md

@@ -331,6 +331,14 @@ for a name instead of using the default.
 > Persistence uses repository storage when the current working directory is inside a git repository.
 > Outside a git repository, sessions fall back to cwd-based storage.
 
+> [!IMPORTANT]
+> Sessions are written as plain JSON under `vim.fn.stdpath("state") .. "/peekstack/"`. Each entry
+> stores the file URI, line/column range, title, provider name, pin/buffer-mode flags, parent
+> popup id, and the timestamp the entry was captured. Each session also tracks `created_at` and
+> `updated_at` metadata. Any path you peek at while persistence is enabled is recorded on disk in
+> cleartext, so avoid enabling persistence on shared machines or for repositories whose file paths
+> or symbol names are sensitive.
+
 ### Auto persist (optional)
 
 When `persist.auto.enabled = true`, peekstack can automatically restore and save a session:

doc/peekstack.txt

@@ -449,6 +449,17 @@ Auto persistence (`persist.auto`) requires `persist.enabled = true`:
 Auto persistence runs only inside a git repository and always uses
 the repository session storage.
 
+PRIVACY NOTE                                      *peekstack-persist-privacy*
+
+Sessions are written as plain JSON under
+`vim.fn.stdpath("state") .. "/peekstack/"`. Each entry stores the file
+URI, line/column range, title, provider name, pin/buffer-mode flags,
+parent popup id, and the timestamp the entry was captured. Each session
+also tracks `created_at` and `updated_at` metadata. Any path you peek at
+while persistence is enabled is recorded on disk in cleartext, so avoid
+enabling persistence on shared machines or for repositories whose file
+paths or symbol names are sensitive.
+
 ==============================================================================
 SETUP RELOAD                                            *peekstack-setup-reload*