fix(subagents): preserve recoverable long results

[tt-a1i]

Summary

• replace head-only subagent result truncation with one shared head+tail projection
• persist the exact over-budget finalText as a private, content-addressed plain-text artifact below Pi's cache
• tell the parent how to page the omitted middle with Pi's native read, without adding another OpenPI tool
• fail closed when artifact persistence is unavailable or encounters a symlink/collision
• update the subagent design document and cover bytes, lines, UTF-8, reuse, failure, and filesystem safety

Closes #64.

Design evidence

Issue #64 contains the full OpenPI/Codex/Hermes comparison and rejected alternatives. The resulting boundary is:

short finalText -> byte-identical delivery, no artifact
long finalText  -> bounded head + tail in context
                -> exact final answer in Pi cache
                -> on-demand recovery through Pi read

No model-facing tool or capability group changes.

Verification

• focused subagent/result tests: 22 pass, 0 fail
• bun run test: 832 Node + 30 Vitest pass, 0 fail
• bun run lint: pass
• bun run typecheck: pass (pre-existing Effect advisory warnings only)
• changed files pass Biome and git diff --check

bun run check still reports one unrelated pre-existing formatting difference in the uncommitted user-owned extensions/sessions/index.ts; this PR does not touch or include that file.

[tt-a1i]

补充动态预算实现与验证证据（commit 3417cab）：

• 借鉴 Hermes 的“按父会话剩余上下文分配批次预算”，但保留 OpenPI 的 75/25 头尾投影与 exact artifact。
• 使用 Pi getContextUsage()，有效读数时取剩余 headroom 的 50%；未知时回退原静态上限。
• 修正完整投影硬上限，包装文字和 artifact 恢复说明也计入预算。
• 专项 26/26；全量 Node 844/844 + Vitest 30/30；lint/typecheck 通过。
• DeepSeek 0731 四子代理烟测：wait 48,773 bytes < 48 KiB，4/4 头尾、4/4 artifact，artifact 0600 且内容哈希一致。

bun run check 唯一阻塞是未纳入本 PR 的用户既有 extensions/sessions/index.ts 格式差异。