fix(providers): CursorAgent --workspace, cwd, and tmp path resolution

[Spencer1O1]

Summary

Pass --workspace on the cursor-agent command. Resolve context.tmp_file under Neovim's cwd (and absolute paths as-is) so cursor-agent writes TEMP_FILE where 99 reads it, especially when Neovim cwd is a parent directory (e.g. C:\Dev with project at C:\Dev\test).

Depends on #185. Review only commit 1629aec. Do not merge until #185 lands.

Closes #181

Verification

Windows

Repro: Neovim :pwd -> C:\Dev, buffer C:\Dev\test\test.js, search "Find the main entrypoint".

Agent targets project tmp, not parent C:\Dev\tmp:

{"path":"C:\\Dev\\test\\tmp/99-1644-prompt","msg":"saved prompt to file"}
{"command":["cursor-agent","--workspace","C:\\Dev","--trust","--force","--model","composer-2.5-fast","--print",...]}
{"msg":"stdout","data":"Task complete. Output written to `C:\\Dev\\test\\tmp\\99-1644`.\n"}

Linux

Repro: Neovim :pwd -> /home, search "Find the main entrypoint".

Command includes --workspace; agent writes the same TEMP_FILE path as in the prompt:

{"path":"/home/spencerls/tmp/99-5795-prompt","msg":"saved prompt to file"}
{"command":["cursor-agent","--workspace","/home","--trust","--force","--model","composer-2.5-fast","--print",...]}
{"msg":"stdout","data":"Task complete. Results are in `/home/spencerls/tmp/99-5795`.\n"}

Test plan

• Provider spec: command includes --workspace
• Manual Windows: Neovim cwd ≠ project — TEMP_FILE paths align
• Manual Linux: --workspace present; agent output path matches TEMP_FILE in prompt