clear dart doc validate-links warnings

Eight unresolved doc references and broken README-rewritten links
that surfaced under `dart doc --validate-links`:

- Library-scope refs `[spawn]`, `[openProject]` qualified to
  `[Backend.spawn]` / `[Backend.openProject]`; same treatment for
  `[resolvePaths]` / `[applyResolved]` on Toolchain.
- `[D-41]` was a decision ID, not a Dart symbol — drop the brackets.
- `[from]` from I18n.interpolated qualified to `[I18nReplacer.from]`.
- `[DefaultSurfaceMap]` was a stale name (private `_defaultSurfaceMap`
  in resolver.dart); switch to backticked path reference since
  dartdoc can't link private members.
- `[icons/]` was a directory, not a symbol; backticked path.
- README links to `legacy/`, `docs/initial-plan.md`, `decisions/`,
  `LICENSE` rewritten as absolute github.com/postmeridiem/clide URLs
  so dartdoc stops re-rooting them into the doc tree.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/builtin/claude/src/claude_session_host.dart

@@ -4,7 +4,7 @@ import 'package:flutter/widgets.dart';
 import 'claude_pane.dart';
 
 /// Hosts the primary Claude pane plus N user-spawned secondary
-/// sessions per [D-41]. Uses [MultitabPane] for the tab strip
+/// sessions per D-41. Uses [MultitabPane] for the tab strip
 /// (drag-reorder, close ×, + button) and [IndexedStack]-mode
 /// keep-alive so switching tabs doesn't tear down the underlying
 /// PTY-backed terminal.