docs: document pre-PR-open branch rebase [doc-updater]

[james-in-a-box]

docs: document pre-PR-open branch rebase in PR phase

Update docs/guides/sdlc-pipeline.md to reflect the new best-effort rebase
step added by d9c416f (#2291, part of #2224 PR 2/3).

The orchestrator now rebases the pipeline branch against current
origin/<base_branch> immediately before calling gh pr create, closing
the gap where base_branch advances during the PR phase. The existing PR
phase section already lists the other pre-PR-open steps (agent-outputs
cleanup, BRC history safety net, draft preservation); this adds a matching
bullet for the new rebase step.

Triggered by: d9c416f (Part of #2224 (2/3): rebase pipeline branch against current base_branch before PR open)

Authored-by: egg

[egg-reviewer]

No agent-mode design concerns. This is a documentation-only change adding a single bullet describing an orchestrator infrastructure step (pre-PR-open rebase). No agent prompts, LLM call paths, or workflow design are touched.

— Authored by egg

[egg-reviewer]

Verified the doc bullet against the implementation in orchestrator/routes/pipelines.py:

• The helper _refresh_pipeline_branch_against_current_base() (line 5760) is invoked at line 7633, immediately before spawner.gateway.create_pr(...) at line 7651 — the bullet's "Immediately before calling gh pr create" is accurate.
• _rebase_pipeline_branch_onto_base (line 5446) is correctly characterized as a phase-start helper; the in-code comment at line 7623–7625 uses the same framing the bullet adopts.
• The helper's docstring (5778–5798) and the seven-step body confirm every claim in the bullet: best-effort (returns False and restores origin/<branch> on every failure path), base_branch is read-only (no push to base ever, only to pipeline_branch), and the PR still opens against the un-rebased tip on failure (the outer try/except at 7641–7648 also catches a hypothetical helper bug so PR creation is never blocked).
• Bullet placement matches execution order: BRC history safety net → pre-PR-open rebase → gh pr create → draft preservation note (which is descriptive, not an action). Style and tone match the existing bullets in the section.

No blocking issues.

— Authored by egg

[james-in-a-box]

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