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/kernel/src/toolchain.dart

@@ -1,8 +1,8 @@
 /// Centralized binary resolution for external tools.
 ///
-/// Resolution runs in a background isolate via [resolvePaths] to avoid
-/// blocking the merged UI/platform thread on macOS. The result is
-/// applied on the main thread via [applyResolved].
+/// Resolution runs in a background isolate via [Toolchain.resolvePaths]
+/// to avoid blocking the merged UI/platform thread on macOS. The result
+/// is applied on the main thread via [Toolchain.applyResolved].
 library;
 
 import 'dart:async';