feat(agent): browse_local_media + import_assets — discover and batch import local media

[nn82pcjt8f-glitch]

Summary / 摘要

Follow-up to #84 and #101. Today the agent can only import local media by exact path (import_asset / import_folder), so it must already know filenames, or ask the user to enumerate them (the exact friction reported in #101). This PR adds a discovery step before importing:

New tools (desktop only, same sandbox boundary as existing import tools):

1. browse_local_media — browse or search local directories without importing: paginated listing (default 100/page), optional query (case-insensitive substring on relative path), kind filter (video/audio/image/gif/svg), recursive (bounded: ≤12 levels, ≤10,000 entries; symlinks not followed). Returns path/name/kind/size/modifiedAt plus nextOffset. Defaults to the home directory.
2. import_assets — batch-import selected paths into the media pool in one call, reusing the existing fingerprint → copy → probe chain and content-duplicate skipping.

Together: browse_local_media (find candidates) → import_assets (import selection) → existing transcription/caption pipeline. No more guessing filenames or pasting 15-file lists into chat.

Design note re: AGENT_IMPORT_ROOTS

One deliberate change: with the discovery tool, local access defaults to enabled (home directory default), and an explicit AGENT_IMPORT_ROOTS acts as a restriction rather than a requirement — the inverse of the current gate. Rationale: #101 showed the "must configure first" path blocks normal users entirely, and discovery is read-only (listing metadata, no bytes imported without a follow-up import_assets). If you prefer to keep whitelist-required semantics, the same tools work unchanged with the gate re-enabled — happy to flip that flag; the plumbing is identical.

Implementation

• shared/agent-local-media.ts — request/result contracts + input validation (path length ≤4096, query ≤256, limit ≤200, offset ≤10k)
• desktop/agent-local-media.ts — main-process browse service (async fs, bounded walk, one IPC channel openchatcut:browse-local-media)
• desktop/agent-path-import.ts — extended for batch import; desktop/main.ts / desktop/preload.ts wire the channel
• src/agent/tools/agent-path-import-tools.ts — tool surface; assets/agent/openchatcut-tool-schemas.json regenerated
• Browser build returns a desktop-only error, consistent with existing import tools

Verify coverage: desktop/agent-local-media.verify.ts, extended desktop/agent-path-import.e2e.verify.ts and src/agent/tools/agent-path-import-tools.verify.ts.

Testing done

• Branch is based on current main tip (19cba6e), clean conflict surface
• 具体场景：纪录片工程目录（数十个 mp4 + 文稿混排）中，agent 先 browse_local_media 按类型/文件名筛选候选，再 import_assets 批量导入，替代人工罗列文件名

Refs: #84, #101