github: make the review link in the PR-body stack footer configurable

[netproteus]

Closes #1409.

sl pr submit hard-codes https://reviewstack.dev/... as the "Best reviewed with" link in the stack footer. Teams on GitHub Enterprise (#512), running a self-hosted ReviewStack fork (#304), or using another stack-aware review tool get a wrong or dead link in every PR. The ReviewStack source itself anticipates alternate hosts (saplingStack.ts: the URL "can change over time (particularly if someone wants to run their own fork of ReviewStack)").

This PR adds two configs:

[github]
pull-request-review-url-template=https://review.example.com/{owner}/{repo}/pull/{number}
pull-request-review-tool-name=MyReview

producing:

Stack created with Sapling. Best reviewed with MyReview.

Design notes

• Unset ⇒ byte-for-byte today's output. The three existing test-ext-github-pr-submit-*.t tests pass unchanged, which doubles as the regression proof.
• The URL template uses str.format placeholders ({owner}, {repo}, {number}, {hostname}) rather than the full Sapling templater: it keeps create_pull_request_title_and_body() a pure, doctest-able function and matches how the URL is already written. Happy to switch to the templater (as github.pr.branch-name-template does) if you'd prefer.
• The sentence shape is deliberately not configurable. parse_stack_information() only tolerates the intro line because it starts with the literal Stack created with [Sapling]; free-form text would also risk corrupting ReviewStack's bullet parsing (* -prefixed lines, bare ---, __->__). Keeping the fixed sentence means even old sl binaries parse customized bodies correctly.
• An invalid URL template aborts at submit time with a message naming the config.
• github.pull-request-include-reviewstack=false still suppresses the whole line; the new configs are no-ops in that case.

Second commit (severable)

parse_stack_information() previously aborted at the first non-bullet line after the footer marker unless it happened to start with "Stack created with [Sapling]" — so hand-editing the footer prose on github.com silently breaks sl pr pull ancestor linking today. Once the modern [//]: # (BEGIN SAPLING FOOTER) marker has been seen, intro lines before the first bullet are now skipped (matching ReviewStack's documented grammar, which treats them as free-text introduction); legacy marker-less bodies keep the conservative behavior. Happy to drop this commit if you'd rather take the config change alone.

Test plan

• New doctests in pull_request_body.py (custom URL, custom name, both, invalid-template abort, round-trip parse of a customized footer, intro-line tolerance, legacy conservatism) — module doctests all pass.
• New eden/scm/tests/test-ext-github-pr-submit-footer.t + tests/github/mock_create_prs_footer.py: the mock server computes the expected footer from the same configs the test sets, so it only matches if the templates were applied. mock_utils.expect_update_pr_request() gains optional review_url/review_tool params (default: existing behavior).
• Existing test-ext-github-pr-submit-{single,overlap,open}.t pass unchanged.
• All four .t tests run green against an OSS make oss-equivalent build (build.py --oss sl) in a manylinux_2_34 container.

🤖 Generated with Claude Code

https://claude.ai/code/session_01UaQdFpGUAEQSBSqHwtQCZ1

[meta-codesync]

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