tools: restructure browser tools and script schemas

- Replace `Action` enum with `Tool` enum using exhaustive switches
- Extract `ScriptIterator` to `Iterator.zig`
- Refactor `schema.zig` into `Schema.zig`
- Move string substitution logic into `tools.zig`
- Clean up `SlashCommand.zig` to only handle REPL meta-commands

src/agent/SlashCommand.zig

@@ -16,26 +16,15 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
-//! REPL-only meta slash commands and re-exports of the PandaScript schema
-//! primitives. The actual slash-command grammar lives in `script/schema.zig`.
+//! REPL-only meta slash commands (`/help`, `/quit`, `/verbosity`). Meta
+//! commands aren't PandaScript — they're handled by `Agent.handleMeta`
+//! and never reach the recorder. PandaScript schema primitives live in
+//! `lp.script.Schema`; consumers should import that directly.
 
 const std = @import("std");
-const lp = @import("lightpanda");
-const schema = lp.script.schema;
 
-pub const SchemaInfo = schema.SchemaInfo;
-pub const ParseError = schema.ParseError;
-pub const Split = schema.Split;
-
-pub const max_hint_slots = schema.max_hint_slots;
-
-pub const globalSchemas = schema.globalSchemas;
-pub const findSchema = schema.findSchema;
-pub const splitNameRest = schema.splitNameRest;
-
-/// Meta slash commands handled directly by Agent.handleMeta.
 pub const MetaCommand = struct {
-    kind: Kind,
+    tag: Tag,
     name: [:0]const u8,
     /// Ghost-text fragment shown after the name + space. Empty when the
     /// command takes no args (`/help`, `/quit`).
@@ -43,13 +32,15 @@ pub const MetaCommand = struct {
     /// Tab-completion candidates for the first positional arg.
     values: []const [:0]const u8,
 
-    pub const Kind = enum { help, quit, verbosity };
+    /// Dispatched by `Agent.handleMeta` via an exhaustive switch so adding
+    /// a new meta command is a compile error until it's wired up there too.
+    pub const Tag = enum { help, quit, verbosity };
 };
 
 pub const meta_commands = [_]MetaCommand{
-    .{ .kind = .help, .name = "help", .hint = "", .values = &.{} },
-    .{ .kind = .quit, .name = "quit", .hint = "", .values = &.{} },
-    .{ .kind = .verbosity, .name = "verbosity", .hint = "<low|medium|high>", .values = &.{ "low", "medium", "high" } },
+    .{ .tag = .help, .name = "help", .hint = "", .values = &.{} },
+    .{ .tag = .quit, .name = "quit", .hint = "", .values = &.{} },
+    .{ .tag = .verbosity, .name = "verbosity", .hint = "<low|medium|high>", .values = &.{ "low", "medium", "high" } },
 };
 
 pub fn findMeta(name: []const u8) ?*const MetaCommand {