Fix four upgrade-path defects the coherence audit found

The ~/.claude.json write was `printf ... > "$CLAUDE_JSON"`, which
truncates before it writes. A write that fails part-way — a full home
volume, which is the exact condition half this release exists to prevent
— leaves the file unparseable, and it never self-heals: the next start's
jq fails on the corrupt file, MERGED is empty, and the guard skips the
write that would have repaired it. That file holds the OAuth account, so
the failure mode is a permanently lost login, in service of a cosmetic
flag that suppresses a tip. Demonstrated: old pattern loses the
credential, new tmp+rename leaves the original intact. The correct
pattern was already in triple-c-task-runner.

The web terminal scoped its xterm key handler to Claude sessions but not
its mobile input bar or its dedicated newline button, so both sent ESC+CR
into `bash -l`, where readline has no binding for it. Silent no-op, and
worse from a button that stays on screen looking live. Both now consult
the active session's type, and the button is disabled with a reason on a
shell tab.

The Config tab claimed "Off overrides a global On" without qualification.
True for the env-var-driven settings, false for TUI mode, Effort level
and Focus mode, whose off state is *removing* a key — an older base
image's entrypoint ignores the instruction to remove it. The copy now
says so and points at the base-image update.

HOW-TO-USE.md said there is no add-task form; AutomationTab renders a
"New task" button. That file is fetched from GitHub at runtime by
help_commands.rs, so the error was live in every user's Help dialog.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBq2rGum6GX7xXgsas1fDc

app/src-tauri/src/web_terminal/terminal.html

@@ -431,6 +431,18 @@
   const mobileInput = document.getElementById('mobileInput');
   const btnEnter = document.getElementById('btnEnter');
   const btnNewline = document.getElementById('btnNewline');
+
+  // Whether the *active* session understands ESC+CR as "insert a newline".
+  //
+  // Only Claude Code does. `bash -l` has no readline binding for `\e\r`, so
+  // sending it there is a silent no-op — which is worse from the mobile bar
+  // than from a hardware key, because the bar puts a dedicated button on
+  // screen that appears to do nothing. The xterm key handler is already scoped
+  // this way; these two paths were not.
+  function activeSessionTakesEscCr() {
+    const s = activeSessionId && sessions[activeSessionId];
+    return !!s && s.type === 'claude';
+  }
   const btnTab = document.getElementById('btnTab');
   const btnCtrlC = document.getElementById('btnCtrlC');
   const scrollBottomBtn = document.getElementById('scrollBottomBtn');
@@ -791,6 +803,7 @@
       switchToSession(remaining[remaining.length - 1]);
     } else {
       activeSessionId = null;
+      syncNewlineButton();
       emptyState.style.display = '';
     }
   }
@@ -817,6 +830,7 @@
 
   function switchToSession(sessionId) {
     activeSessionId = sessionId;
+    syncNewlineButton();
 
     // Update tab styles
     document.querySelectorAll('.tab').forEach(t => t.classList.remove('active'));
@@ -896,7 +910,7 @@
       // reasoning, as the terminal's own key handler above. A hardware
       // keyboard on a tablet is the only way to reach this; the phone case is
       // the dedicated newline button beside Enter.
-      sendTerminalInput(e.shiftKey ? '\x1b\r' : '\r');
+      sendTerminalInput(e.shiftKey && activeSessionTakesEscCr() ? '\x1b\r' : '\r');
     } else if (e.key === 'Tab') {
       e.preventDefault();
       sendTerminalInput('\t');
@@ -904,7 +918,27 @@
   });
 
   btnEnter.onclick = () => { sendTerminalInput('\r'); mobileInput.focus(); };
-  btnNewline.onclick = () => { sendTerminalInput('\x1b\r'); mobileInput.focus(); };
+  btnNewline.onclick = () => {
+    if (!activeSessionTakesEscCr()) { mobileInput.focus(); return; }
+    sendTerminalInput('\x1b\r');
+    mobileInput.focus();
+  };
+
+  // Keep the button's affordance honest: on a shell tab there is no byte that
+  // means "newline without running the line", so the control is disabled
+  // rather than left looking live.
+  function syncNewlineButton() {
+    const usable = activeSessionTakesEscCr();
+    btnNewline.disabled = !usable;
+    btnNewline.title = usable
+      ? 'Insert a newline without submitting (Shift+Enter)'
+      : 'Only Claude sessions support this — a shell runs the line instead';
+  }
+
+  // With no session open yet, `activeSessionTakesEscCr()` is already false —
+  // but nothing had called this, so the button rendered live before the first
+  // tab existed.
+  syncNewlineButton();
   btnTab.onclick = () => { sendTerminalInput('\t'); mobileInput.focus(); };
   btnCtrlC.onclick = () => { sendTerminalInput('\x03'); mobileInput.focus(); };