claude: Bash live-tail detection + read-only file follower (T-325, core)

The detection/follow core for the live-tail sub-card, with the UI wiring
to follow. Claude Code runs every Bash tool itself and clide only sees the
final tool_result block — we can't mirror the running process, so instead
we detect a file-backed source the command follows and open our own
read-only follower on the same file.

- bash_tail_source.dart: detectBashTailSource() parses a Bash command for a
  single, safe, file-backed source (tail/cat/less with one file arg, inside
  the workspace via resolveUnderRoot). Returns null for a pipe-into-tail, a
  redirect, two files, or a path outside the repo — the caller then shows a
  "nothing to follow" note. bashHasTailIntent() gates WHEN the segment
  appears: v1 triggers on `tail`/follow-flags only, so ordinary cat/ls/git
  cards stay clean (cat/less remain detectable for later).
- file_tail_follower.dart: a polling, read-only `tail -f`-style follower
  (no subprocess, no touching Claude's command) that emits the trailing
  window then appended deltas, and re-reads from the top on truncation.

Tested: 19 parser cases (incl. the `git push | tail -25` and outside-
workspace null cases), the intent predicate, and the follower (initial
window / appended delta / missing file / rotation / start / stop).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/claude/src/bash_tail_source.dart

@@ -0,0 +1,119 @@
+/// Detect the file a Bash command follows, for the live-tail sub-card (T-325).
+///
+/// Claude Code runs every Bash tool itself and clide only sees the final
+/// `tool_result` block — we never tap the running command's stdout. So instead
+/// of mirroring the process, we detect a *file-backed source* the command
+/// reads/follows and open our own read-only follower on the same file.
+///
+/// Deliberately conservative (the ticket's "small, explicit allowlist"): only
+/// the read/follow verbs below, only a single file argument, and only paths
+/// that resolve INSIDE the workspace. Anything else — a pipe into `tail`, a
+/// redirect, two files, a path outside the repo — returns null so the caller
+/// shows a "nothing to follow" affordance instead of following the wrong thing.
+library;
+
+import 'dart:io';
+
+import 'package:clide/src/files/path_safety.dart';
+
+/// Verbs whose single file argument clide can independently follow read-only.
+const Set<String> _followVerbs = {'tail', 'cat', 'less'};
+
+/// The file [command] reads/follows that clide can mirror read-only, as an
+/// absolute path inside [workspaceRoot] — or null when there is no single,
+/// safe, file-backed source. See the library doc for the policy.
+String? detectBashTailSource(String command, {required Directory workspaceRoot}) {
+  String? found;
+  for (final segment in _commandSegments(command)) {
+    final tokens = _tokenize(segment);
+    if (tokens.isEmpty || !_followVerbs.contains(tokens.first)) continue;
+    final files = _fileArgs(tokens.first, tokens.sublist(1));
+    if (files.length != 1) continue; // 0 → reads stdin (a pipe); >1 → ambiguous
+
+    final String resolved;
+    try {
+      resolved = resolveUnderRoot(workspaceRoot, files.single);
+    } on PathOutsideRoot {
+      continue; // outside the workspace → don't follow (v1 policy)
+    }
+    if (found != null && found != resolved) return null; // two distinct sources
+    found = resolved;
+  }
+  return found;
+}
+
+/// Whether [command] expresses an intent to FOLLOW a file — used to decide
+/// when to surface the live-tail segment at all, so ordinary commands (`ls`,
+/// `git status`, a plain `cat`) get no segment, but a `tail …` with no
+/// followable file still shows the "nothing to follow" note. v1 triggers on
+/// `tail` or a follow flag (`-f`/`-F`/`--follow`); `cat`/`less` are detectable
+/// sources but don't trigger the UI on their own (T-325).
+bool bashHasTailIntent(String command) {
+  for (final segment in _commandSegments(command)) {
+    final tokens = _tokenize(segment);
+    if (tokens.isEmpty) continue;
+    if (tokens.first == 'tail') return true;
+    if (tokens.any((t) => t == '-f' || t == '-F' || t == '--follow')) return true;
+  }
+  return false;
+}
+
+/// Split a command line into command/pipeline segments on `|`, `;`, `&`. The
+/// doubled forms (`&&`, `||`) fall out as empty middles and are dropped.
+Iterable<String> _commandSegments(String command) => command.split(RegExp(r'[|;&]')).where((s) => s.trim().isNotEmpty);
+
+/// Positional (non-flag) file arguments for [verb]. Skips flags, consumes the
+/// value of `tail -n N` / `-c N`, honours `--` (end of options), and stops at a
+/// redirect (`>` / `<`) — everything after a redirect targets a fd, not the
+/// command's input.
+List<String> _fileArgs(String verb, List<String> args) {
+  final files = <String>[];
+  for (var i = 0; i < args.length; i++) {
+    final a = args[i];
+    if (a == '--') {
+      files.addAll(args.sublist(i + 1).where((t) => !t.contains('>') && !t.contains('<')));
+      break;
+    }
+    if (a.contains('>') || a.contains('<')) break; // a redirect ends positional args
+    if (a.startsWith('-')) {
+      if (verb == 'tail' && (a == '-n' || a == '-c') && i + 1 < args.length) i++; // -n N / -c N
+      continue;
+    }
+    files.add(a);
+  }
+  return files;
+}
+
+/// Minimal shell tokeniser: splits on whitespace, honours single/double quotes
+/// (no escape or expansion handling — enough to recover file arguments).
+List<String> _tokenize(String s) {
+  final out = <String>[];
+  final buf = StringBuffer();
+  String? quote;
+  var has = false;
+  for (var i = 0; i < s.length; i++) {
+    final ch = s[i];
+    if (quote != null) {
+      if (ch == quote) {
+        quote = null;
+      } else {
+        buf.write(ch);
+      }
+      has = true;
+    } else if (ch == '"' || ch == "'") {
+      quote = ch;
+      has = true;
+    } else if (ch == ' ' || ch == '\t') {
+      if (has) {
+        out.add(buf.toString());
+        buf.clear();
+        has = false;
+      }
+    } else {
+      buf.write(ch);
+      has = true;
+    }
+  }
+  if (has) out.add(buf.toString());
+  return out;
+}