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/backend.dart

@@ -1,8 +1,8 @@
 /// Manages the backend isolate lifecycle.
 ///
 /// Two-phase boot:
-///   1. [spawn] — starts the isolate, resolves toolchain (binary checks only).
-///   2. [openWorkspace] — initializes services for a specific project root.
+///   1. [Backend.spawn] — starts the isolate, resolves toolchain (binary checks only).
+///   2. [Backend.openProject] — initializes services for a specific project root.
 library;
 
 import 'dart:async';
@@ -34,7 +34,7 @@ class Backend {
   int _validateId = 0;
 
   /// Spawn the backend isolate. Returns when the toolchain is resolved.
-  /// No services are active yet — call [openWorkspace] to activate.
+  /// No services are active yet — call [openProject] to activate.
   static Future<Backend> spawn({
     required IsolateClient Function(SendPort backendPort) clientFactory,
     String? hintRoot,