fix(attribution): unify research-session format on "(self-directed)" suffix

Resolves the format inconsistency between the forward fix and the 304-row
backfill. Both halves now produce prs.submitted_by = "rio (self-directed)":

- research-session.sh: drop proposed_by from the frontmatter template.
  extract.py path 1 (proposed_by-driven) no longer fires; path 2 fires
  instead and constructs f"{agent} (self-directed)" — matches backfill.

- attribution.py: normalize_handle now strips "(self-directed)" suffix
  immediately after lowercase+@-strip, before alias lookup. Closes the
  phantom-person-event class on any future replay through
  record_contributor_attribution. Round-trips through alias rules keyed
  on bare agent names.

Test (5 cases) still passes; suffix-strip behavior verified against
hostile inputs (whitespace, casing, mid-string occurrences must NOT
match — only trailing pattern).

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

lib/attribution.py

@@ -82,6 +82,7 @@ def normalize_handle(handle: str, conn=None) -> str:
     if not handle:
         return ""
     h = handle.strip().lower().lstrip("@")
+    h = re.sub(r"\s*\(self-directed\)\s*$", "", h)
     if conn is None:
         return h
     try:

research/research-session.sh

@@ -267,7 +267,6 @@ format: tweet | thread
 status: unprocessed
 priority: high | medium | low
 tags: [topic1, topic2]
-proposed_by: ${AGENT}
 intake_tier: research-task
 ---