fix(fs): resolve paths against the caller's session cwd

The ACP bridge gives each session its own workspace (SessionHeader.cwd), and
dsh-tool-bash already resolves a bash workdir against it. But ctx.fs.resolve(path)
took no caller context and dsh-fs-local resolved every relative path against a
fixed config.cwd (process.cwd() at plugin load) — so in the ACP demo `write
foo.txt` and `bash cat foo.txt` hit different directories the moment an editor
opens any project other than the server's launch dir.

Thread the session cwd into resolution, mirroring dsh-tool-bash: widen
FileSystem.resolve to resolve(path, opts?: { cwd?: string }); dsh-fs-local bases
a relative path on opts.cwd ?? config.cwd (absolute paths ignore it); the
read/write/edit tools derive it via a shared sessionCwd(exec) helper
(exec.agent?.session.header.cwd). The provider stays free of dsh-agent/dsh-session
— the tool projects exec → cwd and hands over a plain string, per the
explicit-at-seams convention. Backward compatible (the arg is optional).

Tests: fs-local resolve(path,{cwd}) bases relative on the passed cwd / ignores it
for absolute; tool integration writes/reads/edits in a session cwd != config.cwd
and verifies the file on disk (proven to fail on the pre-fix no-cwd path). Fakes
that stood in a bare {session:{}} now carry a header so sessionCwd doesn't throw.
RFC in docs/rfc/implemented/architecture/2026-07-02-fs-per-session-cwd.md.

packages/fs/tool-fs/src/read.ts

@@ -20,6 +20,7 @@ import type {} from '@deepseek-ai/dsh-fs'
 import type {} from '@deepseek-ai/dsh-system-prompt'
 import { buildWindow, formatReadOutput } from './read-render.ts'
 import type { FileReadOutcome } from './read-render.ts'
+import { sessionCwd } from './session-cwd.ts'
 
 /** Default and maximum number of lines returned by one `read` call. */
 export const READ_LIMIT = 2000
@@ -68,7 +69,8 @@ export function applyReadTool(ctx: Context): void {
     },
     async execute(args, exec): Promise<ContentBlock[]> {
       const input = parseReadArgs(args)
-      const target = await ctx.fs.resolve(input.filePath)
+      const cwd = sessionCwd(exec)
+      const target = await ctx.fs.resolve(input.filePath, cwd !== undefined ? { cwd } : undefined)
 
       // One stat: type check + size routing + the version recorded as observed.
       // A writer racing between this stat and the read can at worst make a LATER