feat(msg): /msg skill — read/write cross-session mailbox by @id

Thin skill wrapper over the existing kei-message jsonl mailbox so the
user (and agents) can talk between Claude Code sessions with @id syntax:
  /msg                 read my inbox (to me or "all")
  /msg @frontend text  send to a session (identity = its cwd basename)
  /msg all text        broadcast
  /msg list | who      whole bus / known recipients

- kei-message.sh send now accepts a leading @name as the recipient
  (first token only; a later @x stays literal). --to still works.
- skills/msg/SKILL.md documents the identity model (cwd-basename),
  pull delivery (recipient's next turn via mailbox-inject hook), and the
  first-contact discovery path (who / all).
- README skills count 68 -> 69.

Verified: @name/all/--to parsing (3 cases) + end-to-end send/inbox/who
via the live script in a sandbox HOME. Skill registered + discoverable.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -8,7 +8,7 @@ OpenClaw / Kimi from the same source-of-truth.
 
 **Apache 2.0** — explicit patent grant + retaliation clause. 105 Rust
 crates [REAL: `grep -E '^\s*"[a-z-]+",' _primitives/_rust/Cargo.toml | wc -l`],
-68 skills [REAL: `ls skills/ | wc -l`], 54 hooks
+69 skills [REAL: `ls skills/ | wc -l`], 54 hooks
 [REAL: `ls hooks/*.sh | wc -l`], 38 agent manifests
 [REAL: `ls _manifests/*.toml | wc -l`], 85 substrate blocks
 [REAL: `find _blocks/ -name '*.md' | wc -l`], 18 capability atoms
@@ -83,7 +83,7 @@ The web installer (`web-install.sh` in this repo, served at
 repo and delegates to `bootstrap.sh` — single source of truth, no
 duplicated install logic.
 
-38 agents + 68 skills + 54 hooks + nightly consolidation wired in
+38 agents + 69 skills + 54 hooks + nightly consolidation wired in
 ~60 seconds. Twelve install profiles (`outcome-only`, `minimal`,
 `core`, `frontend`, `ops`, `dev`, `mcp`, `cortex`, `local-mirror`,
 `dashboard`, `full-hub`, `full`) defined in

scripts/kei-message.sh

@@ -32,6 +32,9 @@ case "$cmd" in
         --to)   to="$2";   shift; shift ;;
         --from) me="$2";   shift; shift ;;
         --)     shift; body="$body $*"; break ;;
+        # Leading @name = recipient (e.g. `send @frontend hi`). First token only;
+        # a later @x stays literal text.
+        @?*)    if [ "$to" = "all" ] && [ -z "$body" ]; then to="${1#@}"; else body="$body $1"; fi; shift ;;
         *)      body="$body $1"; shift ;;
       esac
     done

skills/msg/SKILL.md

@@ -0,0 +1,62 @@
+---
+name: msg
+description: Read or write the cross-session mailbox by @id. Send a message to another Claude Code session (`/msg @name text`), read your own inbox (`/msg` with no args), broadcast to everyone (`/msg all text`), list the whole bus, or discover who is reachable. Thin wrapper over the `kei message` jsonl mailbox — messages land in the recipient's NEXT turn via the mailbox-inject hook (pull, not push). Use whenever the user wants sessions/agents to talk to each other.
+argument-hint: "[@name] <message>  |  (empty = read inbox)  |  list  |  who"
+---
+
+# /msg — Inter-Session Mailbox
+
+A persistent append-only bus so ANY Claude Code session can message ANY other —
+not just Agent-Teams teammates, no tmux, no daemon. Backed by
+`~/.claude/scripts/kei-message.sh` writing `~/.claude/mailbox/messages.jsonl`.
+The `mailbox-inject.sh` UserPromptSubmit hook pulls each session's unread into
+its context once per turn, so delivery is **pull** (arrives on the recipient's
+next turn), not instant push.
+
+## Identity model
+
+- **Your address** = the basename of this session's working directory (`$PWD`).
+  So a session running in `~/Projects/frontend` is reachable as `@frontend`.
+- **`all`** is the broadcast channel — every session sees `to:"all"` messages.
+- You can override the sender with `--from <name>` and the reader identity with
+  `--me <name>` if a session's cwd basename isn't the name you want to use.
+
+## Command map
+
+Interpret `$ARGUMENTS` and run the matching command via Bash, then show its
+output to the user. The launcher `kei message …` and the script path are
+equivalent — prefer the script path (always present after install):
+
+| User typed | Run |
+|---|---|
+| `/msg` (no args) | `~/.claude/scripts/kei-message.sh inbox` |
+| `/msg @frontend ship it` | `~/.claude/scripts/kei-message.sh send @frontend ship it` |
+| `/msg all standup in 5` | `~/.claude/scripts/kei-message.sh send all standup in 5` |
+| `/msg list` | `~/.claude/scripts/kei-message.sh list` |
+| `/msg who` (or `channels`) | `~/.claude/scripts/kei-message.sh channels` |
+
+Rules for parsing `$ARGUMENTS`:
+
+1. **Empty** → read inbox (`inbox`). Show the messages addressed to this
+   session or to `all`.
+2. **Starts with `@<name>`** → send to that recipient; the rest is the body.
+   A `@x` that appears later in the body stays literal text.
+3. **Starts with `all `** → broadcast; the rest is the body.
+4. **`list`** → print the recent whole bus (every from→to line).
+5. **`who` / `channels`** → print known recipient names (use this to discover
+   who is reachable before sending the first message).
+6. Anything else with no leading `@`/`all` → treat as a broadcast body, OR ask
+   the user who the recipient is if it's ambiguous.
+
+## Discovery (first-message problem)
+
+A recipient only appears in `who` after it has sent or been sent a message, so
+for the very first contact either broadcast with `all`, or ask the user for the
+target session's cwd-basename. Don't invent a recipient name.
+
+## Notes
+
+- Sending never blocks and never notifies the recipient out-of-band — they see
+  it on their next turn. For a time-sensitive ping, tell the user it's queued.
+- This is plain files: `cat ~/.claude/mailbox/messages.jsonl` is the raw bus.
+- Bypass the inject hook for a session with `KEI_MAILBOX_BYPASS=1`.