Minor docs updates

[skarim]

• Corrects the README, docs, CLI help, and agent skill to describe stack pushes as non-atomic, with explicit per-branch --force-with-lease checks and partial-success retry behavior.
• Clarifies that gh stack push pushes only active branches — excluding merged and queued branches — across the README, gh stack push help text, and agent skill.
• Documents the 100-PR stack limit in the FAQ.
• Documents that the pull_request.opened webhook never carries a stack object (a PR is added to a stack only after it's opened) and that the stacked action is the event that signals stack membership — added an intro note and strengthened the stacked section in the Webhooks reference, plus a new FAQ entry.

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[Copilot]

Pull request overview

Updates documentation to accurately explain non-atomic stack pushes and the 100-PR limit.

Changes:

• Documents per-branch force-with-lease behavior.
• Clarifies partial push outcomes and retries.
• Adds the 100-PR stack limit.

Show a summary per file

File	Description
skills/gh-stack/SKILL.md	Updates agent guidance for push and submit.
README.md	Revises push behavior documentation.
docs/src/content/docs/reference/cli.md	Updates CLI push reference.
docs/src/content/docs/guides/workflows.md	Clarifies rebase/push workflow behavior.
docs/src/content/docs/faq.md	Documents the stack size limit.
cmd/push.go	Corrects push command help and comment.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 3
• Review effort level: Medium