feat(agy): Google Antigravity CLI hook integration

hooks/antigravity/README.md

@@ -2,8 +2,90 @@
 
 > Part of [`hooks/`](../README.md) — see also [`src/hooks/`](../../src/hooks/README.md) for installation code
 
-## Specifics
+## Supported Modes
 
-- Prompt-level guidance only (no programmatic hook) -- relies on Antigravity reading custom instructions
-- `rules.md` contains the instruction to prefix all shell commands with `rtk`, usage examples, and meta commands
-- Installed to `.agents/rules/antigravity-rtk-rules.md` (project-local) by `rtk init --agent antigravity`
+RTK supports two separate Google Antigravity integrations:
+
+### 1. Antigravity CLI (`agy`) — Programmatic Hook
+
+The `agy` CLI (successor to Gemini CLI) supports a JSON hook protocol.  RTK registers as a `PreToolUse` hook that rewrites shell commands before execution.
+
+**Install (global):**
+```bash
+rtk init -g --agent agy
+```
+
+**Install (project-local):**
+```bash
+rtk init --agent agy
+```
+
+**Hook location:**
+- Global: `~/.agents/hooks.json`
+- Project: `.agents/hooks.json`
+
+**Hook entry written:**
+```json
+{
+  "hooks": {
+    "PreToolUse": [
+      {
+        "type": "command",
+        "command": "rtk hook antigravity",
+        "toolNameMatcher": "^(run_command|Bash)$"
+      }
+    ]
+  }
+}
+```
+
+**JSON protocol:**
+
+Input (`run_command` tool):
+```json
+{"toolCall": {"name": "run_command", "args": {"CommandLine": "git status"}}}
+```
+
+Output (rewritten):
+```json
+{"decision": "allow", "toolCall": {"args": {"CommandLine": "rtk git status"}}}
+```
+
+Input (`Bash` tool):
+```json
+{"toolCall": {"name": "Bash", "args": {"command": "cargo test"}}}
+```
+
+Output (rewritten):
+```json
+{"decision": "allow", "toolCall": {"args": {"command": "rtk cargo test"}}}
+```
+
+No output is produced for non-shell tools or commands that don't match any RTK filter — the tool runs unchanged.
+
+**Uninstall:**
+```bash
+rtk init --uninstall --agent agy          # project-local
+rtk init -g --uninstall --agent agy       # global
+```
+
+### 2. Antigravity IDE — Prompt-Level Guidance
+
+The Antigravity IDE (formerly "Antigravity") reads project rules files.  RTK installs its awareness instructions as a rules file.
+
+**Install (project-local only):**
+```bash
+rtk init --agent antigravity
+```
+
+- Installs `.agents/rules/antigravity-rtk-rules.md` in the project root
+- Instructs Antigravity to prefix shell commands with `rtk`
+- No programmatic hook — relies on the model following the rules
+
+## Gemini CLI (Legacy)
+
+The Gemini CLI is still supported but is now considered **legacy**.  New users should use `agy`.
+
+```bash
+rtk init -g --gemini    # Gemini CLI (legacy)
+```

src/cmds/system/json_cmd.rs

@@ -106,7 +106,8 @@ fn compact_json(value: &Value, depth: usize, max_depth: usize) -> String {
         Value::Number(n) => format!("{}{}", indent, n),
         Value::String(s) => {
             if s.len() > 80 {
-                let end = s.floor_char_boundary(77);
+                let mut end = 77.min(s.len());
+                while !s.is_char_boundary(end) { end -= 1; }
                 format!("{}\"{}...\"", indent, &s[..end])
             } else {
                 format!("{}\"{}\"", indent, s)

src/cmds/system/pipe_cmd.rs

@@ -140,9 +140,9 @@ fn find_wrapper(input: &str) -> String {
 }
 
 pub fn auto_detect_filter(input: &str) -> fn(&str) -> String {
-    let end = input.len().min(1024);
+    let mut end = input.len().min(1024);
     // Avoid panic: byte 1024 may fall inside a multi-byte UTF-8 char
-    let end = input.floor_char_boundary(end);
+    while !input.is_char_boundary(end) { end -= 1; }
     let first_1k = &input[..end];
 
     if first_1k.contains("test result:") && first_1k.contains("passed;") {

src/hooks/constants.rs

@@ -21,6 +21,13 @@ pub const OPENCODE_PLUGIN_FILE: &str = "rtk.ts";
 pub const CURSOR_DIR: &str = ".cursor";
 pub const CODEX_DIR: &str = ".codex";
 pub const GEMINI_DIR: &str = ".gemini";
+
+#[allow(dead_code)]
+pub const ANTIGRAVITY_DIR: &str = ".agents";
+
+/// Config subdirectory for the Antigravity CLI (agy) under ~/.gemini/
+pub const AGY_CLI_SUBDIR: &str = "antigravity-cli";
+
 pub const HERMES_DIR: &str = ".hermes";
 pub const HERMES_PLUGINS_SUBDIR: &str = "plugins";
 pub const HERMES_PLUGIN_NAME: &str = "rtk-rewrite";