Add fx session fork and fx session rewind (stack 1/2)

README.md

@@ -87,6 +87,17 @@ fx session resume last
 fx session resume --id <id>
 ```
 
+`fx session <id>` prints the saved conversation with a `[turn N]` label above every turn. Those labels are the boundaries the branch and undo commands take:
+
+```bash
+fx session fork <id> --at 7
+fx session rewind <id> --by 2
+```
+
+`fork` copies the first 7 turns into a brand-new session and prints the new ID to resume. The source session is left exactly as it was. `rewind` drops the last 2 turns from the session in place, keeping its ID. Both accept `--id <id>` and `--json`.
+
+Rewound turns are not erased. The rewind is recorded as a new revision in the session's event log, and the files those turns referenced stay on disk.
+
 Each interactive session names its terminal tab. The title prefers the session name, falls back to the workspace name, and keeps the active model as secondary context. Renaming or resuming a session updates the tab, and exiting clears the fx-owned title. Noninteractive commands do not emit terminal-title controls.
 
 Run `/feedback` to open the feedback form at `fx.sh/feedback`. It does not create a diagnostic or change the clipboard.

scripts/pgso/corpus.json

@@ -113,6 +113,7 @@
   ],
   "verification_scenarios": [
     {"name": "verify-auto-mode-reliability", "argv": ["bun", "test", "--max-concurrency", "1", "./auto-mode-reliability.test.ts"], "test_file": "auto-mode-reliability.test.ts"},
+    {"name": "verify-session-fork", "argv": ["bun", "test", "--max-concurrency", "1", "./session-fork.test.ts"], "test_file": "session-fork.test.ts", "requires_tmux": false},
     {"name": "verify-oauth-keychain-migration", "argv": ["bun", "test", "--max-concurrency", "1", "./oauth-keychain-migration.test.ts"], "test_file": "oauth-keychain-migration.test.ts", "allow_keychain": true},
     {"name": "verify-tui-auth-source-selection", "argv": ["bun", "test", "--max-concurrency", "1", "./tui-auth-source-selection.test.ts"], "test_file": "tui-auth-source-selection.test.ts"},
     {"name": "verify-tui-composer-edit-contracts", "argv": ["bun", "test", "--max-concurrency", "1", "./tui-composer-edit-contracts.test.ts"], "test_file": "tui-composer-edit-contracts.test.ts"},

src/builtins/commands.zig

@@ -193,14 +193,16 @@ pub const top_level_specs = [_]TopLevelSpec{
     .{
         .kind = .session,
         .token = "session",
-        .usage = "session <last|id>|--id <id> [--json] | session resume [last|<id>] | session resume --id <id> | session migrate <id>|--id <id> [--allow-large] [--json] | session recover <id>|--id <id> [--json]",
-        .summary = "Inspect, resume, migrate, or recover saved sessions",
+        .usage = "session <last|id>|--id <id> [--json] | session resume [last|<id>] | session resume --id <id> | session migrate <id>|--id <id> [--allow-large] [--json] | session recover <id>|--id <id> [--json] | session fork <id>|--id <id> --at <turn> [--json] | session rewind <id>|--id <id> --by <count> [--json]",
+        .summary = "Inspect, resume, migrate, recover, fork, or rewind saved sessions",
         .options = &.{
             .{ .flag = "last", .description = "Inspect the current workspace session" },
             .{ .flag = "--id <id>", .description = "Inspect a saved session by exact id" },
             .{ .flag = "resume [last|<id>]", .description = "Resume the latest workspace session or a session by id" },
             .{ .flag = "migrate <id>", .description = "Migrate a saved session to the current format" },
             .{ .flag = "recover <id>", .description = "Copy a recoverable corrupt session into a new session" },
+            .{ .flag = "fork <id> --at <turn>", .description = "Branch a session into a new session holding its first <turn> turns" },
+            .{ .flag = "rewind <id> --by <count>", .description = "Drop the last <count> turns from a session in place" },
             .{ .flag = "--allow-large", .description = "Permit migrating an oversized session" },
             json_option,
         },
@@ -312,6 +314,8 @@ pub const top_level_help_groups = [_]TopLevelHelpGroup{
         .{ .usage = "session resume [last|id]", .summary = "Resume the latest workspace session or a session by id" },
         .{ .usage = "session migrate <id>", .summary = "Migrate a saved session to the current format" },
         .{ .usage = "session recover <id>", .summary = "Copy a recoverable corrupt session" },
+        .{ .usage = "session fork <id> --at <turn>", .summary = "Branch a session at a turn into a new session" },
+        .{ .usage = "session rewind <id> --by <count>", .summary = "Drop the last turns from a session in place" },
     } },
     .{ .entries = &.{
         .{ .kind = .login, .usage = "login [vercel|codex|grok]" },