feat(path-picker): show hidden files and folders, and harden the secret blocklist

The picker behind Link Existing's "Browse" and the mobile keyboard's Path key
refused every path with a dot-prefixed segment, so `.github/workflows/ci.yml`
could not be selected and a hidden folder could not be opened at all. It gains
the same `.*` toggle as the File Viewer: default OFF, per-device, and applied to
both the listing and the preview endpoint, which re-resolves the path
independently.

That dotfile filter was quietly doing security work. The picker's roots include
Home, so with every hidden path unreachable the shared blocklist never had to
name the credentials that live in dot-directories. Lifting the filter removes
that accident, so `isSensitivePath` now covers them explicitly: SSH keys at any
depth rather than only under $HOME, GPG keyrings, AWS/GCloud/Azure/Docker/
Kubernetes credentials, npm, Yarn, git, gh, netrc, PyPI, RubyGems, Cargo and
Terraform tokens, .pgpass and .my.cnf, and the Claude and Codeman agent
credentials. `~/.codeman/` and `~/.claude/` stay attachable as trees, since the
publish skill and the review-card loop read from them; only their secret-bearing
members are named.

Everything else still applies with the toggle on: blocked trees, sensitive
files, root confinement, ownership scoping and symlink-escape checks. A hidden
entry whose realpath is a secret is dropped from the listing, and opening it is
refused.

Follows #221

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/schemas.ts

@@ -65,6 +65,14 @@ const filesystemPickerPathSchema = z
   })
   .refine((p) => !p.split('/').includes('..'), { message: 'Path traversal is not allowed' });
 
+/**
+ * Opt-in flag for listing dot-prefixed entries in the path picker. Absent means
+ * off, so an old client keeps the previous behavior. It is a string rather than
+ * a boolean because it arrives as a query parameter; `'false'` is accepted (and
+ * means off) so a client can send the flag unconditionally.
+ */
+const showHiddenQuerySchema = z.enum(['true', 'false']).optional();
+
 /** Query validation for the lazy, allowlisted filesystem path picker. */
 export const FilesystemBrowseQuerySchema = z.object({
   path: filesystemPickerPathSchema.optional(),
@@ -73,6 +81,7 @@ export const FilesystemBrowseQuerySchema = z.object({
     .max(100)
     .regex(/^[a-zA-Z0-9_-]+$/, 'Invalid session id')
     .optional(),
+  showHidden: showHiddenQuerySchema,
 });
 
 /** Query validation for a single allowlisted path-picker file preview. */
@@ -83,6 +92,7 @@ export const FilesystemPreviewQuerySchema = z.object({
     .max(100)
     .regex(/^[a-zA-Z0-9_-]+$/, 'Invalid session id')
     .optional(),
+  showHidden: showHiddenQuerySchema,
 });
 
 /**