fix(sandbox): preserve filesystem cwd semantics

.agents/notes/implemented/architecture/2026-07-02-fs-per-session-cwd.md

@@ -8,13 +8,15 @@ The ACP bridge gives every session its own workspace: `session/new` records the
 
 Filesystem resolution used one plugin-load cwd while bash used the session project directory. Relative paths therefore disagreed whenever the editor project differed from the server launch directory; snapshots hid the bug by making those paths identical.
 
+A valid absolute cwd can itself have two apparent parents: when it contains `symlink/..`, filesystem lookup follows the symlink before applying `..`, while `path.resolve()` erases both components lexically. Resolving sandbox policy lexically while launching bash from the raw cwd granted the unrelated lexical parent, denied writes in the real workspace, and let filesystem tools resolve relative paths into the wrong directory.
+
 ## Decision
 
-Thread the caller's session cwd into path resolution, exactly as `dsh-tool-bash` already does for `workdir`. The **caller** (the tool) supplies the cwd; the provider does not read a session or agent.
+Thread the caller's session cwd into path resolution, exactly as `dsh-tool-bash` already does for `workdir`. Resolve that cwd to its native filesystem identity before any lexical join, and reuse the resolved sandbox-policy root for mutations and sandboxed bash calls so one call has one workspace identity. The **caller** (the tool) supplies the cwd; the provider does not read a session or agent.
 
 - `FileSystem.resolve` accepts `resolve(path: string, opts?: { cwd?: string; signal?: AbortSignal }): Promise<FsTarget>`. `opts.cwd` is the base a RELATIVE `path` resolves against; an absolute `path` ignores it; omitting `opts.cwd` uses the backend's own default. `opts.signal` cancels resolution when the backend performs I/O. The options object keeps both caller-owned resolution controls together without positional growth.
 - `dsh-fs-local.resolve` uses `resolveLocalTarget(opts?.cwd ?? this.config.cwd, path)`. `config.cwd` stays the default for a caller that supplies none (non-ACP / no-session use, and the single-session stdio demo where `process.cwd()` IS the workspace).
-- `dsh-tool-fs`'s `read`/`write`/`edit` derive the session cwd through a shared `sessionCwd(exec)` helper (`exec.agent?.session.header.cwd`, mirroring bash's `resolveWorkdir`) and pass it to `resolve`. A non-agent / headerless caller yields `undefined`, so the backend applies its default.
+- `dsh-tool-fs`'s `read`/`write`/`edit` derive the session cwd through a shared `sessionCwd(exec)` helper (`exec.agent?.session.header.cwd`, mirroring bash's `resolveWorkdir`), canonicalize it with native realpath semantics, and pass it to `resolve`. A sandboxed mutation reuses the complete policy's `workspaceRoot`; a non-agent / headerless caller yields `undefined`, so the backend applies its default.
 
 ## Alternatives considered
 
@@ -27,6 +29,7 @@ The default lives in ONE place — the provider's `config.cwd`. `sessionCwd` ret
 ## Consequences
 
 - In the ACP demo the fs tools and bash now agree on each session's workspace; an editor can open any project folder and both tool families act on it.
+- A session cwd containing `symlink/..` resolves to the same physical workspace for bash launch, relative filesystem paths, and the sandbox grant; the lexical parent receives no grant.
 - No change to `FsTarget` identity: `targetKey` is still the realpath of the resolved absolute path, so observed-state keying and symlink identity are unaffected — a correct per-session cwd produces the same key bash targets.
 - Backward compatible: every existing `resolve(path)` call (all in tests) keeps working; the new argument is optional.
 - The single-session stdio demo is unaffected: it supplies no session cwd (its agent's session has no `cwd`), so resolution falls back to `config.cwd = process.cwd()`, which is the workspace.