docs: prose pass, merge-mark wording fix, refreshed social card

[lesnik512]

Prose pass over the user-facing docs (README.md and the mkdocs-published pages) to remove AI-writing patterns. No code, code blocks, inline code, commands, or link targets changed.

What changed

• Em/en dashes: 47 → 0 across README.md, docs/index.md, docs/providers/{github,gitlab}.md, docs/strategies/{branch-prefix,conventional-commits}.md. Replaced with colons, commas, parentheses, periods, or rewrites.
• Bold run-in labels: 34 → 0. Blockquote callouts ("Required setup.", "First tag.", "Masking caveat.", etc.) are now plain sentences; the GitHub/GitLab token-scope lists and README strategy/"Built with" lists no longer open with bold labels; the troubleshooting bullets on both provider pages are now one ### heading per error with a plain paragraph under it.
• Contrasts and filler: "rather than" / clipped tails reworded where they only added weight; "resilient" (README) and "appropriate" dropped; all-caps MUST/NOT lowercased.
• Title case: ## Quick Start → ## Quick start on both provider pages (slug quick-start unchanged) and the mkdocs nav label.

Fact fix: merge-commit detection

docs/strategies/branch-prefix.md said the strategy only fires on subjects containing Merge branch, then listed a GitHub Merge pull request subject as matching. The paragraph now names both default marks. Evidence, semvertag/strategies/branch_prefix.py:

merge_mark_texts: tuple[_NonEmptyStr, ...] = pydantic.Field(
    default=("Merge branch", "Merge pull request"),

and decide() checks any(mark in subject for mark in self.config.merge_mark_texts).

Social card

docs/assets/social-card.png is replaced with the card regenerated from the current project description in modern-python/.github#125.

Verification

• just lint-ci: pass
• just test: 475 passed
• mkdocs build --strict (docs/requirements.txt): pass