Add plan-mode endpoint to agentic path

POST /api/chat {mode:'plan'} returns a structured step list from the LLM
without executing any tools. The client can show and edit this plan before
sending the real agentic request.

Implements issue #27 item 2 (Agent panel soul contract — plan mode).

chat.el

@@ -1573,6 +1573,55 @@ fn next_bridge_id() -> String {
     return "br-" + uid
 }
 
+fn handle_chat_plan(body: String) -> String {
+    let message: String = json_get(body, "message")
+    if str_eq(message, "") {
+        return "{\"error\":\"message required\",\"plan\":null}"
+    }
+
+    let req_model: String = json_get(body, "model")
+    let model: String = if str_eq(req_model, "") { chat_default_model() } else { req_model }
+
+    let op_home: String = env("HOME")
+    let op_user: String = env("USER")
+    let op_display: String = if str_eq(op_user, "") { "the current user" } else { op_user }
+
+    // Compile context — same intent-seeding as agentic path so the plan is grounded.
+    let ctx: String = engram_compile(message)
+    let ctx_block: String = if str_eq(ctx, "") { "" } else { "\n\n[CONTEXT]\n" + ctx }
+
+    let plan_system: String = "You are in PLAN MODE. Your job is to produce a concise step-by-step plan for the request below — WITHOUT executing it.\n\nReturn ONLY a JSON object. No markdown. No preamble. No explanation. Just the JSON:\n{\"steps\":[{\"id\":\"s1\",\"title\":\"<2-6 word title>\",\"detail\":\"<one concrete sentence>\"},{\"id\":\"s2\",...}]}\n\nPlan rules:\n- 3-7 steps (more only when genuinely needed for a complex multi-file task)\n- Each step is one atomic, independently verifiable action\n- title: 2-6 words, imperative (e.g. \"Read config file\", \"Write updated handler\")\n- detail: exactly one sentence describing what happens\n- No tool calls. No execution. No side effects. The user approves before anything runs.\n\nOperator: " + op_display + " at " + op_home + ctx_block
+
+    let raw: String = llm_call_system(model, plan_system, message)
+
+    let is_error: Bool = str_starts_with(raw, "{\"error\"")
+    if is_error {
+        return "{\"error\":\"plan generation failed\",\"plan\":null,\"detail\":" + raw + "}"
+    }
+
+    // Extract the JSON object from the response (LLM sometimes wraps in markdown).
+    let brace_start: Int = str_index_of(raw, "{")
+    // Scan backwards to find the last closing brace (str_last_index_of not available).
+    let brace_end: Int = -1
+    let scan_i: Int = str_len(raw) - 1
+    while scan_i >= 0 {
+        let ch: String = str_slice(raw, scan_i, scan_i + 1)
+        let brace_end = if str_eq(ch, "}") && brace_end < 0 { scan_i } else { brace_end }
+        let scan_i = if brace_end >= 0 { -1 } else { scan_i - 1 }
+    }
+    let plan_json: String = if brace_start >= 0 {
+        if brace_end > brace_start {
+            str_slice(raw, brace_start, brace_end + 1)
+        } else {
+            raw
+        }
+    } else {
+        raw
+    }
+
+    return "{\"plan\":" + plan_json + ",\"model\":\"" + json_safe(model) + "\"}"
+}
+
 fn handle_chat_agentic(body: String) -> String {
     let message: String = json_get(body, "message")
     if str_eq(message, "") {