Support session-scoped external folder refs / 支持会话级外部文件夹引用

[SivanCola]

Summary

• Register folders dropped from outside the workspace as session-scoped workspace references instead of inserting plain absolute-path text.
• Keep the Composer UI readable by showing real display paths while submitting internal ref tokens that the controller resolves for the current session.
• Support on-demand external folder subpaths: @<external-folder-token>/sub/file resolves to file content, nested directories can be listed, and file search/autocomplete can return registered external-folder children without exposing raw local paths as submit text.
• Make folder references explicit and bounded: directory refs inject a capped listing only, note that file contents are not inlined, skip common generated/vendor folders, and tell the model to request listed files when content is needed.
• Let read-only built-in tools (read_file, ls, grep, glob) resolve registered external-folder token paths so the agent can inspect/search authorized external children on demand without adding those folders as writable workspace roots.
• Extend workspace previews/open/reveal paths so authorized external ref search results can be inspected through the same desktop UI surfaces.

References

Refs #5370, #5447

Contribution Credits

• Kept/adapted: feat: 拖拽文件夹到输入框自动插入路径文本 #5447 by @ttmouse identified the folder-drop regression and implemented the initial path-text insertion direction. This PR keeps that UX goal but replaces path-text insertion with a session-scoped folder-reference mechanism plus on-demand child-path resolution.
• Out of scope: the font-size/custom scaling request in [Feature]: 请求恢复文件夹直接拖入功能，以及自定义字号功能 #5370 remains a separate follow-up.

Cache Impact

Cache-impact: low - touches cache-sensitive boot/tool files, but only to wire runtime path resolution for session-authorized external folder refs; no stable prompt text, model-visible tool schema, or provider request serialization changes.
Cache-guard: go test ./internal/tool/... includes schema-stability coverage for workspace tools with an external read resolver; go test ./internal/boot covers boot wiring.
System-prompt-review: reviewed in this PR; the boot change passes a runtime read-path resolver into the controller/tools and does not alter system prompt composition.

Verification

• go test ./internal/tool/...
• go test ./internal/control
• go test ./internal/boot
• go test . -run 'TestFileRefs|TestAttachDropped|TestWorkspaceRelativeIn|TestIsImageExt' in the desktop module
• pnpm --dir desktop/frontend typecheck
• pnpm --dir desktop/frontend exec tsx src/__tests__/at-matches.test.ts
• pnpm --dir desktop/frontend exec tsx src/__tests__/composer-goal-toggle.test.tsx
• git diff --check