feat(board): materialize program items as Main-PM roots, make reports actionable (#711)

Two coupled gaps in the Board Program output path.

Approved items were created unowned and in BACKLOG. Nothing dispatches
BACKLOG, and once activated a cell PM claimed the parentless task as a root,
where _cell_pm_complete resolves its merge target through
resolve_parent_branch — which for a parentless task falls through to the
project head rung. The result was a cell branch merging straight into the
trunk, bypassing the Main-PM root, the root->master PR and the CEO gate
(live: PRs #703 and #704 both targeted slave directly).

All eight materializers now create a PENDING, main-pm-assigned root with
team=Team.MAIN_PM, matching what approve_and_start does for an intake draft.
The team is load-bearing, not cosmetic: _next_hint_pr_fail,
_deliver_pr_fail_to_owner, delegate's wave-chain dispatch and the PR layer
label all key on it, and a cell-teamed root drops the 'do NOT re-submit the
root' steer that exists because of PR #138's infinite pr_fail loop. The
item's own cell survives as a delegation hint in the description, which is
what the Main PM's briefing renders.

Periscope, Sentinel and Coroner produced artifacts with no way to act on
them — three panel surfaces carried explicit 'no approve/reject UI' comments
while each item already held a machine-readable suggested action. They now
have per-item approve and dismiss, modelled on the roadmap queue: idempotent
per item, CEO-gated, deep-copy-before-mutate so SQLAlchemy's dirty check
still fires, and every decision recorded through record_decision so it
reaches the next cycle's prompt. Approving materializes through the same
corrected Main-PM-owned path.

Target project resolves to each engine's own existing anchor — RoboCo's
project for Periscope and Sentinel, the incident's project for Coroner — and
fails with a clean invalid_state naming what is unresolvable rather than
guessing at a repo.

Co-authored-by: Renn F <rennf93@users.noreply.github.com>

tests/unit/gateway/test_content_actions_periscope.py

@@ -424,6 +424,11 @@ async def test_propose_market_brief_persists_and_completes_the_exploration_task(
     assert payload["headline"] == "A rival tool shipped agentic PR review this week"
     assert len(payload["findings"]) == len(findings)
     assert payload["findings"][0]["id"] == "finding-0"
+    # Each finding still carries its own per-item CEO decision (Periscope
+    # Service.approve_finding/reject_finding) even though the exploration
+    # task completes here.
+    assert payload["findings"][0]["status"] == "proposed"
+    assert payload["findings"][0]["materialized_task_id"] is None
     assert payload["threats"] == ["Feature parity gap"]
     assert payload["opportunities"] == ["Lean into structured findings"]
     assert payload["positioning_note"] == "Emphasize the findings ledger in messaging"