rebase with preserve dates opt

[skarim]

Add --committer-date-is-author-date flag to gh stack rebase

Introduce an opt-in --committer-date-is-author-date flag (with
--preserve-dates alias) for gh stack rebase. The flag is passed
through to every underlying git rebase invocation in the cascade,
keeping committer dates equal to author dates so that identical content
rebased onto an identical parent produces stable SHAs. This reduces
spurious force-push notifications and noisy review timelines, especially
in deep stacks where bottom branches get re-rebased on every merge.

Git layer changes:

• Add RebaseOpts struct with CommitterDateIsAuthorDate field to
  internal/git/gitops.go
• Update Ops interface, defaultOps, public wrappers, and
  rebaseContinueOnce/tryAutoResolveRebase helpers to accept and
  forward the flag
• Update MockOps to match the new signatures

Command layer changes:

• Register --committer-date-is-author-date and --preserve-dates
  flags on the cobra command in cmd/rebase.go
• Add CommitterDateIsAuthorDate to cascadeRebaseOpts and thread it
  to all git.Rebase/git.RebaseOnto calls in cmd/utils.go
• Persist the flag in rebaseState JSON so --continue resumes with
  the same behavior; pass it to RebaseContinue and subsequent cascade
  calls
• Update internal/modify/apply.go callers to pass zero-value
  RebaseOpts{}

Tests:

• Update all existing mock signatures in rebase, sync, and modify tests
• Add tests for flag passthrough, --preserve-dates alias, state
  round-trip, --continue flag restoration, and conflict state
  persistence

Docs:

• Update flag tables and examples in README.md and
  docs/src/content/docs/reference/cli.md

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