fix(buzz-acp): ground the agent workspace on Windows

[sumit-m]

Problem

workspace_section decides whether to emit the [Workspace] grounding block by testing cwd.starts_with('/'). No Windows path satisfies that, so on Windows the section is always None: agents are never told their working directory, nor that AGENTS.md, PLANS/, RESEARCH/, WORK_LOGS/, OUTBOX/ and REPOS/ live there.

The cwd itself is set correctly, so files still land in the right place by default — but the prompt loses the grounding that exists precisely to stop the agent hunting around $HOME for those directories.

Change

• Use Path::is_absolute for the absoluteness test. It is target-aware, so C:\Users\me\.buzz qualifies. The explicit cwd != "/" check stays, because is_absolute("/") is true on Unix and that value is the current_dir() failure sentinel.
• Join REPOS with Path::join rather than interpolating a literal /, so a Windows nest reads …\.buzz\REPOS instead of a mixed-separator path.

Tests

The existing tests hardcoded /Users/me/.buzz with no cfg gate, which is why this went unnoticed — on Windows they asserted nothing meaningful. They now go through a cfg'd SAMPLE_ABS_CWD so the suite exercises the platform it runs on, plus a cross-platform regression guard and one rejection case per platform (C:foo drive-relative on Windows, a Windows path on Unix).

cargo clippy -p buzz-acp --all-targets clean; pool::tests 98/98 on Windows.