feat(coding-agent): wire the filesystem tools into the demo

Load dsh-fs-local + dsh-fs-policy + dsh-tool-fs after tool-todo (mirroring the
acp-agent wiring), and steer the system prompt to prefer read/write/edit for
file ops with bash for shell/tests/search. Update the welcome line and the
FIXME(config-comments) bash note.

Doc sweep now that both demos ship the fs tools and the seam resolves per-session
cwd: architecture.md and the event-gate RFC no longer say the demos do file ops
through bash / that no config wires the tools; the coding-agent + examples
READMEs and the AGENTS.md layout blurb list the fs tools; the acp-agent README
drops the launch-dir caveat (per-session cwd now works, so the server can launch
anywhere).

(stdio-agent is single-session, so fs-local's cwd = process.cwd() is the
workspace. Keyless boot smoke is blocked locally by an unrelated inotify
watcher-limit ENOSPC that also hits demo:echo; the config parses and the same fs
stack boots green in the acp-agent snapshot tier.)

examples/coding-agent/cordis.yml

@@ -30,7 +30,7 @@
 
 # Local bash executor for agent-core's tool-bash schema.
 # FIXME(config-comments): keep this executor note from implying bash is the
-# whole tool set; subagent and todo_write are loaded below.
+# whole tool set; filesystem, subagent, and todo_write are loaded below.
 - id: bash
   name: '@deepseek-ai/dsh-bash-local'
   config:
@@ -46,16 +46,17 @@
     # under ./.sessions); unset starts a fresh session each run.
     resumeSessionId: !!js process.env.RESUME_SESSION_ID
     persistenceRoot: './.sessions'
-    welcome: 'coding-agent ready. Give it a coding task (its tools are bash, subagent, and todo_write).'
+    welcome: 'coding-agent ready. Give it a coding task (its tools are read, write, edit, bash, subagent, and todo_write).'
     systemPrompt: |
       You are coding-agent, a CLI coding assistant.
 
-      Your tools are bash (plus bash_output/bash_kill for background
-      tasks) and subagent. Do ALL file operations through bash: read with
-      cat/sed/head, search with grep, write with heredocs (cat <<'EOF' >
-      file), edit with sed or a rewrite. Each bash call runs in a fresh
-      shell — pass workdir instead of cd, and never rely on shell state
-      between calls.
+      Your tools are read/write/edit for file operations, bash (plus
+      bash_output/bash_kill for background tasks), and subagent. Use read to
+      inspect UTF-8 text files, write to create or replace files, and edit for
+      targeted literal replacements. Use bash for shell commands, tests,
+      searches, and operations that are not ordinary file reads or edits. Each
+      bash call runs in a fresh shell — pass workdir instead of cd, and never
+      rely on shell state between calls.
 
       Use the subagent tool to delegate a focused, self-contained subtask
       to a fresh child agent (it works in its own context and returns only
@@ -123,3 +124,17 @@
 # session log (todo/write), rendered as a stdio checklist / ACP plan.
 - id: tool-todo
   name: '@deepseek-ai/dsh-tool-todo'
+
+# Filesystem capability stack: local provider, read-before-write/edit policy
+# gate, then the model-facing read/write/edit tools. stdio-agent is a single
+# session, so relative paths resolve from the process cwd (the workspace).
+- id: fs-local
+  name: '@deepseek-ai/dsh-fs-local'
+  config:
+    cwd: !!js process.cwd()
+
+- id: fs-policy
+  name: '@deepseek-ai/dsh-fs-policy'
+
+- id: tool-fs
+  name: '@deepseek-ai/dsh-tool-fs'