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.

[sumit-m]

Flagging an overlap I should have caught before opening this: #3130 (opened 3 days earlier) changes the same predicate in workspace_section.

Both PRs replace cwd.starts_with('/') with Path::is_absolute(), which is the substantive half of this fix — a Windows nest at C:\Users\me\.buzz-dev is not /-rooted, so the [Workspace] section was never emitted at all.

#3130 is the better version of that half. It also rejects filesystem roots generally via path.parent().is_some(), covering C:\, \?\C:\ and UNC shares rather than just /, and it documents why the platform-dependent behaviour of is_absolute() is correct here rather than a bug to revert.

What is unique to this PR is the REPOS path separator:

let repos = std::path::Path::new(cwd).join("REPOS");

Interpolating {cwd}/REPOS emits C:\Users\me\.buzz-dev/REPOS — a mixed-separator path handed to an agent that is explicitly told not to go looking elsewhere. #3130 leaves that line untouched, so merging it alone fixes emission but still ships the mixed separator.

Happy to go whichever way suits: rebase this onto #3130 so it becomes separator-only, or fold the root check from #3130 into this one and close that. No preference beyond not leaving two PRs racing on one function.