fix: resolve 5 remaining pre-publish blockers (14, 15, 17, 21, 25c)

- completion-promise-detector: restrict to assistant text parts only,
  remove tool_result from completion detection (blocker 14)
- ralph-loop tests: flip tool_result completion expectations to negative
  coverage, add false-positive rejection tests (blocker 15)
- skill tools: merge nativeSkills into initial cachedDescription
  synchronously before any execute() call (blocker 17)
- skill tools test: add assertion for initial description including
  native skills before execute() (blocker 25c)
- docs: sync all 4 fallback-chain docs with model-requirements.ts
  runtime source of truth (blocker 21)

Verified: bun test (4599 pass / 0 fail), tsc --noEmit clean

src/tools/skill/tools.test.ts

@@ -621,7 +621,7 @@ describe("skill tool - nativeSkills integration", () => {
     const tool = createSkillTool({
       skills: [createMockSkill("seeded-skill")],
       nativeSkills: {
-        async all() {
+        all() {
           return [{
             name: "native-visible-skill",
             description: "Native skill exposed from config",
@@ -629,13 +629,14 @@ describe("skill tool - nativeSkills integration", () => {
             content: "Native visible skill body",
           }]
         },
-        async get() { return undefined },
-        async dirs() { return [] },
+        get() { return undefined },
+        dirs() { return [] },
       },
     })
 
     //#when
     expect(tool.description).toContain("seeded-skill")
+    expect(tool.description).toContain("native-visible-skill")
     await tool.execute({ name: "native-visible-skill" }, mockContext)
 
     //#then