fix(fleet): always bake runtime-bin into pane PATH (ignore launcher PATH)

The previous `_build_runtime_bin_prefix()` skipped candidate dirs that
were already present in the LAUNCHER process's \$PATH.  This is wrong:
the tmux pane inherits the tmux SERVER environment, not the launcher's
env.  A dir on the launcher's \$PATH may be absent from the server env,
so the prefix could come back empty and the pane would fail with
'command not found'.

Remove the `case ":${PATH}:"` check that tested against the live launcher
PATH.  Keep the existence check (`[ -d "$dir" ]`) and the dedup-within-
the-constructed-prefix guard.  The pane command's `export PATH="<prefix>:${PATH}"`
harmlessly absorbs any overlap with the server PATH.

Add test 5 to test-start-agent-session.sh: sets FAKE_RUNTIME_BIN5 on the
launcher's \$PATH and asserts it still appears in the generated pane PATH
export — directly guarding this regression.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RMoEx7hfdFGjUiCHuN1RRi

packages/mosaic/framework/tools/fleet/start-agent-session.sh

@@ -32,8 +32,12 @@ fi
 #   2. $(npm config get prefix)/bin  (if npm is on PATH)
 #   3. Fallbacks: $HOME/.npm-global/bin and $HOME/.local/bin
 #
-# Only directories that already exist are included.  Directories already
-# present in PATH are skipped so we never duplicate entries.
+# Only directories that already exist are included.  The prefix is baked into
+# the pane command regardless of what the LAUNCHER process's $PATH contains,
+# because the tmux pane inherits the tmux SERVER environment (not this script's
+# environment).  A dir on the launcher's PATH may be absent from the server PATH,
+# so every existing candidate must always be included.  Dedup within the
+# constructed prefix avoids listing the same dir twice.
 _build_runtime_bin_prefix() {
   local candidates=()
 
@@ -55,14 +59,11 @@ _build_runtime_bin_prefix() {
   local prefix=""
   for dir in "${candidates[@]}"; do
     [ -d "$dir" ] || continue
-    case ":${PATH}:" in
-      *":${dir}:"*) continue ;;  # already present
-    esac
     if [ -z "$prefix" ]; then
       prefix="$dir"
     else
       case ":${prefix}:" in
-        *":${dir}:"*) ;;  # already in our prefix
+        *":${dir}:"*) ;;  # already in our prefix — skip
         *) prefix="${prefix}:${dir}" ;;
       esac
     fi