feat: filter agent-restricted skills from prompts and tool description

Skills with an `agent` frontmatter field are intended for a specific
agent. Previously they still appeared in:
- every agent's system prompt (via `buildAvailableSkills`)
- the `skill` tool's `<available_items>` description visible to all agents

This wasted tokens and could mislead agents into attempting calls that
would be rejected at execution time.

Changes:
- `buildAvailableSkills`: new optional `agentName` parameter; when
  provided, skills whose `definition.agent` does not match are excluded
- `builtin-agents.ts`: pass per-agent name to `buildAvailableSkills`
  for sisyphus, hephaestus, and atlas, so each agent's prompt only
  lists the skills it is allowed to use
- `createSkillTool` (`tools.ts`): exclude agent-restricted skills from
  both the eager and lazy description builds, keeping the shared tool
  description free of skills the current agent cannot access

Execution-time enforcement (throwing on mismatch) is unchanged; this
change adds the earlier, description-level visibility gate.

Tests: new `available-skills.test.ts` (5 cases) + 3 new cases in
`tools.test.ts` covering the description-filter and execute paths.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/agents/builtin-agents.ts

@@ -100,8 +100,6 @@ export async function createBuiltinAgents(
     description: categories?.[name]?.description ?? CATEGORY_DESCRIPTIONS[name] ?? "General tasks",
   }))
 
-  const availableSkills = buildAvailableSkills(discoveredSkills, browserProvider, disabledSkills, teamModeEnabled)
-
   // Collect general agents first (for availableAgents), but don't add to result yet
   const { pendingAgentConfigs, availableAgents } = collectPendingBuiltinAgents({
     agentSources,
@@ -129,7 +127,7 @@ export async function createBuiltinAgents(
     systemDefaultModel,
     isFirstRunNoCache,
     availableAgents,
-    availableSkills,
+    availableSkills: buildAvailableSkills(discoveredSkills, browserProvider, disabledSkills, teamModeEnabled, "sisyphus"),
     availableCategories,
     mergedCategories,
     directory,
@@ -148,7 +146,7 @@ export async function createBuiltinAgents(
     systemDefaultModel,
     isFirstRunNoCache,
     availableAgents,
-    availableSkills,
+    availableSkills: buildAvailableSkills(discoveredSkills, browserProvider, disabledSkills, teamModeEnabled, "hephaestus"),
     availableCategories,
     mergedCategories,
     directory,
@@ -171,7 +169,7 @@ export async function createBuiltinAgents(
     availableModels,
     systemDefaultModel,
     availableAgents,
-    availableSkills,
+    availableSkills: buildAvailableSkills(discoveredSkills, browserProvider, disabledSkills, teamModeEnabled, "atlas"),
     mergedCategories,
     directory,
     userCategories: categories,