Link the agent half of user/ on the Mac mini

Manifest lines can now name an OS. The SSH config, pwswitch and espanso are
Linux only, so link-user on the Mac links CLAUDE.md, rules, skills and the
opencode commands and nothing else. link-mcp no longer aborts under the Mac's
bash 3.2 when a server has no token.

user/manifest

@@ -1,6 +1,6 @@
 # What in user/ goes where, and in what form.
 #
-#   <kind>  <source under user/>  <destination>
+#   <kind>  <source under user/>  <destination>  [os]
 #
 # kind is one of:
 #   link   symlink the destination at the source, so editing either edits the
@@ -14,6 +14,10 @@
 #   copy   copy once, and never touch it again if the destination exists. Use
 #          this for files a machine legitimately diverges on.
 #
+# os is optional: linux or darwin, matched against `uname -s`. A line naming
+# one is skipped everywhere else. Leave it off for anything every machine
+# wants, which is most of this file; the Mac mini only takes the agent half.
+#
 # Lines beginning with # are comments and blank lines are ignored. ~ in a
 # destination is this user's home.
 #
@@ -43,21 +47,22 @@ link  agents/rules       ~/.claude/rules
 # ── SSH ──────────────────────────────────────────────────────────────────────
 # Host aliases only. Keys are per-machine and are never tracked; this is the
 # file that makes `ssh server` work the moment a machine is set up, and it is
-# what the SSH Hosts launcher command reads.
-link  ssh/config         ~/.ssh/config
+# what the SSH Hosts launcher command reads. Linux only: it names id_ed25519
+# and the desktop keyring's agent socket, and the Mac has neither.
+link  ssh/config         ~/.ssh/config  linux
 
 # ── Scripts ──────────────────────────────────────────────────────────────────
 # Personal commands wanted on every machine, desktop or server. ~/.local/bin is
 # on PATH in both shell configs. pwswitch toggles passwordless sudo through a
 # sudoers.d drop-in, so an agent can run sudo and the password comes back after.
-link  bin/pwswitch       ~/.local/bin/pwswitch
+link  bin/pwswitch       ~/.local/bin/pwswitch  linux
 
 # ── Espanso ──────────────────────────────────────────────────────────────────
 # The personal expansion triggers. Copied rather than linked, because this is a
 # file each machine may add its own triggers to. It lands before setup-identity
 # runs, and that stage only seeds a file that does not exist yet, so what is
 # tracked here wins over the name and email the interview would have written.
-copy  espanso/identity.yml  ~/.config/espanso/match/identity.yml
+copy  espanso/identity.yml  ~/.config/espanso/match/identity.yml  linux
 
 # ── OpenCode ─────────────────────────────────────────────────────────────────
 # Claude Code carries its commands and agents as plugins, which opencode cannot