fix(buzz-acp): loosen workspace-scan guardrail to allow named paths (#6261)

## Problem

Agents refused to read a specific, user-named path outside `.buzz` (e.g.
`~/.claude/skills/context-health-check/SKILL.md`), and behaved
inconsistently — refusing in one channel, listing the same dir in
another. This is the issue Aaron and Wes hit:
buzz://message?channel=e62570dd-33ad-42c5-b92b-75f2689f9694&id=401199157f8a9734109fdddfbbce4603a1ff465fae895042f29738ad52a5d15f

Root cause: two prompt fragments told agents not to search `$HOME` "or
other directories". The intent (added in #1194 by Will Pfleger) was
narrow — stop unscoped `$HOME` sweeps that fire macOS TCC dialogs or
make a model invent its own workspace dir. But "or other directories"
reads as a blanket filesystem wall, so a cautious model generalized it
to "any path outside `.buzz`" and refused a targeted, named read.

## Fix

Prompt-only. Reframe both fragments per context-engineering guidance —
state the reason and trust judgement, rather than banning a technique:

**`crates/buzz-acp/src/pool.rs` — `workspace_section()`**
> …This is where you already are, so start here rather than scanning
`$HOME`. Any specific path the user names is fine to read.

**`crates/buzz-acp/src/base_prompt.md` line 106**
> These paths are relative to your working directory — start there for
your own files rather than scanning `$HOME` or `/`. When the user names
a specific path, read it.

Preserves the original anchor intent (start at the grounded workspace;
avoid unscoped `$HOME` sweeps) while explicitly permitting a named-path
read — the exact case that was broken.

## Testing

`cargo test -p buzz-acp` — 816 unit + 9 integration tests pass, 0
failures. Prompt-only change; the doc comment at `pool.rs:1327-1333`
already states the narrow intent and is left as-is.

Signed-off-by: Salman Mohammed <smohammed@squareup.com>
Co-authored-by: Leo <5faf251baee50ee6bcde338aef6acdd70bb3e60115664c2cd490d94a55dfc488@buzz.block.builderlab.xyz>

crates/buzz-acp/src/base_prompt.md

@@ -103,7 +103,7 @@ Your persistent workspace is in your working directory:
 
 Knowledge files use `ALL_CAPS_WITH_UNDERSCORES.md` naming. `AGENTS.md` lists active agents and roles. See `AGENTS.md` in your working directory for full workspace conventions.
 
-These paths are relative to your working directory — keep exploration there. Never run `find` or recursive searches over `$HOME` or `/` hunting for workspace files: they live under your working directory, not elsewhere on disk.
+These paths are relative to your working directory — start there for your own files rather than scanning `$HOME` or `/`. When the user names a specific path, read it.
 
 ## Agent Memory
 

crates/buzz-acp/src/pool.rs

@@ -1657,8 +1657,8 @@ fn workspace_section(cwd: &str) -> Option<String> {
             "[Workspace]\nYour absolute working directory is `{cwd}`. All workspace \
              files — `AGENTS.md`, `RESEARCH/`, `PLANS/`, `GUIDES/`, `WORK_LOGS/`, \
              `OUTBOX/` — and any repositories you clone (under `{cwd}/REPOS/`) live \
-             here. This is where you already are; do not search `$HOME` or other \
-             directories for them."
+             here. This is where you already are, so start here rather than scanning \
+             `$HOME`. Any specific path the user names is fine to read."
         ))
     } else {
         None