fix(install): refuse --path only for tools that would overwrite each other; re-land installer test suite (#772)

Re-lands the install.sh regression suite from #772 (reverted in #827) together
with the guard change that makes it pass, so CI goes green in one step.

Background. #825 made --path refuse more than one --tool, on #819's report that
several tools sharing one destination clobber each other. That was right in
spirit and over-broad in practice, and it was implemented without verifying
the premise. Measured by installing one agent with every tool into a sandbox
and comparing what landed:

  <division>-<slug>.md  (raw copy)   claude-code, copilot
  <slug>.md             (converted)  gemini-cli, opencode, qwen, zcode
  agency-<slug>/SKILL.md             antigravity, osaurus

Tools in the same group write identical filenames and silently overwrite each
other (qwen + gemini-cli lose a file while both print [OK]). Tools in different
groups coexist (claude-code + codex, claude-code + qwen). Every other tool's
output is distinct.

The guard now refuses --path only for a colliding pair, naming both tools and
the reason, and allows the rest. path_collision_group() holds the measured
table; re-measure if a converter's naming changes.

The suite's two-tool --path cases used claude-code + copilot, which collide:
the "installs exactly one agent" count of 1 was passing because copilot had
overwritten claude-code's identical file. They now use claude-code + codex and
assert that BOTH outputs survive, which a single count cannot show; a new case
asserts the colliding pair is refused. codex has no committed output, so those
cases convert (only the two raw-copiers work under --no-convert in a fresh
checkout).

Verified under bash 3.2 (the macOS CI leg): 29 passed, 0 failed, 1 xfail.
Guard spot-checked against all three measured groups plus a cross-group pair.

Suite, workflow and CONTRIBUTING note by @SergiorCode (#772). Refs #772 #819
#825 #827.

Co-Authored-By: SergiorCode <SergiorCode@users.noreply.github.com>
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

scripts/install.sh

@@ -283,6 +283,20 @@ install_file() {
 }
 
 # resolve_dest <tool> <default> — --path > $ENV_VAR > default.
+# path_collision_group <tool> — tools in the same group write identical
+# filenames into a shared --path and would overwrite each other; empty means
+# the tool's output is distinct and may share a path with anything. Derived by
+# installing one agent with every tool into a sandbox and comparing what
+# landed; re-measure if a converter's output naming changes.
+path_collision_group() {
+  case "$1" in
+    claude-code|copilot)             printf 'raw-source-md' ;;  # <division>-<slug>.md
+    gemini-cli|opencode|qwen|zcode)  printf 'slug-md' ;;        # <slug>.md
+    antigravity|osaurus)             printf 'agency-skill' ;;   # agency-<slug>/SKILL.md
+    *)                               printf '' ;;
+  esac
+}
+
 resolve_dest() {
   local tool="$1" def="$2" var=""
   [[ -n "$OVERRIDE_PATH" ]] && { printf '%s' "$OVERRIDE_PATH"; return; }
@@ -1258,11 +1272,22 @@ main() {
       $duplicate || _cleaned+=("$_t")
     done
     _tool_list=("${_cleaned[@]}")
-    # --path is a single-destination override; with several tools every one of
-    # them would land in the same directory and clobber each other.
+    # --path is one shared directory. Tools that write the same filenames into
+    # it silently overwrite each other; tools with distinct outputs coexist.
+    # Refuse only the colliding combinations (see path_collision_group).
     if [[ -n "$OVERRIDE_PATH" && ${#_tool_list[@]} -gt 1 ]]; then
-      err "--path sets ONE destination; use it with exactly one --tool (got ${#_tool_list[@]}: ${_tool_list[*]})."
-      exit 1
+      local _ta _tb _ga _gb
+      for _ta in "${_tool_list[@]}"; do
+        _ga="$(path_collision_group "$_ta")"; [[ -z "$_ga" ]] && continue
+        for _tb in "${_tool_list[@]}"; do
+          [[ "$_tb" == "$_ta" ]] && continue
+          _gb="$(path_collision_group "$_tb")"
+          if [[ "$_ga" == "$_gb" ]]; then
+            err "--path is one shared directory, and $_ta and $_tb write the same filenames into it — they would overwrite each other. Use one of them per --path (tools with distinct outputs may share one)."
+            exit 1
+          fi
+        done
+      done
     fi
   fi