pr-merge --base-line: gated intra-line exception (B5) (#1471)

Co-authored-by: marcie <[email protected]>

packages/mosaic/framework/tools/git/pr-merge.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 # pr-merge.sh - Merge pull requests on Gitea or GitHub
-# Usage: pr-merge.sh -n PR_NUMBER [-m squash] [-d] [--expect-head SHA] [--no-ci-expected] [--co-author-trailers --escalate-to PRINCIPAL]
+# Usage: pr-merge.sh -n PR_NUMBER [-m squash] [-d] [--expect-head SHA] [--no-ci-expected] [--base-line BRANCH] [--co-author-trailers --escalate-to PRINCIPAL]
 
 set -euo pipefail
 
@@ -30,6 +30,12 @@ Options:
     -d, --delete-branch     Delete the head branch after merge
         --dry-run           Run metadata/login preflight without merging
         --expect-head SHA   Refuse unless the PR head matches this full commit SHA
+        --base-line BRANCH  Documented intra-line exception (B5, ruled
+                            2026-08-29): authorize a merge whose base is
+                            neither main nor next (stacked PR lines). The
+                            value must MATCH the PR base; all other gates
+                            (queue guard, head pin, CI) still run and the
+                            exception is recorded in the merge audit.
         --no-ci-expected    Assert the target repository has no CI: forward --no-ci-expected to the queue guard (requires repository admin)
         --co-author-trailers Build verified trailers from linked PR commit authors
         --escalate-to NAME  Named principal for an unresolved-author BLOCK
@@ -46,6 +52,7 @@ EOF
 }
 
 # Parse arguments
+BASE_LINE_OVERRIDE=""
 while [[ $# -gt 0 ]]; do
     case $1 in
         -n|--number)
@@ -64,6 +71,11 @@ while [[ $# -gt 0 ]]; do
             DRY_RUN=true
             shift
             ;;
+        --base-line)
+            [[ $# -ge 2 ]] || { echo "Error: --base-line requires a branch name." >&2; exit 1; }
+            BASE_LINE_OVERRIDE="$2"
+            shift 2
+            ;;
         --expect-head)
             if [[ $# -lt 2 ]]; then
                 echo "Error: --expect-head requires one full commit SHA." >&2
@@ -172,8 +184,17 @@ if [[ "$DECL_STATE" == valid && "$DECL_SCHEMA" == 2 ]]; then
 else
     repo_decl_warn_absent_irreversible "pr-merge"
     if [[ "$BASE_BRANCH" != "main" && "$BASE_BRANCH" != "next" ]]; then
-        echo "Error: Mosaic policy allows merges only for PRs targeting 'main' or 'next' (found '$BASE_BRANCH')." >&2
-        exit 1
+        if [[ -n "$BASE_LINE_OVERRIDE" && "$BASE_LINE_OVERRIDE" != "$BASE_BRANCH" ]]; then
+            echo "Error: --base-line '$BASE_LINE_OVERRIDE' does not match the PR base '$BASE_BRANCH' (refusing; the exception must name the real base)." >&2
+            exit 1
+        fi
+        if [[ "$BASE_LINE_OVERRIDE" == "$BASE_BRANCH" ]]; then
+            echo "audit: base-line exception — merge into '$BASE_BRANCH' authorized by explicit --base-line (B5 ruling 2026-08-29); queue guard, head pin, and CI gates unchanged." >&2
+        else
+            echo "Error: Mosaic policy allows merges only for PRs targeting 'main' or 'next' (found '$BASE_BRANCH')." >&2
+            echo "  A ruled intra-line merge may pass --base-line '$BASE_BRANCH' (same gates; the exception is recorded)." >&2
+            exit 1
+        fi
     fi
 fi
 if [[ -z "$HEAD_BRANCH" || -z "$HEAD_REPO" || ! "$HEAD_SHA" =~ ^[0-9a-fA-F]{40}$ ]]; then