feat: reword non-HEAD commit messages, and add Combine Commits action

[rkaiser0324]

Extends the existing "Edit Message" action to support non-HEAD commits (previously rejected with "Editing commit messages for non-HEAD commits is not yet supported."), and adds a new "Combine Commits..." action that squashes a range of commits (selected via the existing compare/expand commit UI) into one, with a user-supplied message.

Both are implemented as a single interactive rebase, driven by two small standalone Node helper scripts (compiled to out/rebaseSequenceEditor.js and out/rebaseMessageEditor.js) set as GIT_SEQUENCE_EDITOR and GIT_EDITOR - the message is collected via the existing dialog UI before the rebase starts, so no editor window is opened and no interactive rebase state is left for the user to resolve on cancel.

UI changes

• "Edit Message..." (existing) now works for any commit, not just HEAD.
• New "Combine Commits..." action, gated behind a combineCommits context-menu-visibility setting (same convention as editMessage) — also backfilled the two missing editMessage/combineCommits entries in the package.json settings schema.
• The Combine Commits dialog's textarea defaults to all the original commit messages concatenated (oldest first).
• Both dialogs are now double-width/double-height ("large") — previously bounded by a fixed 360px width. Applied automatically to any dialog with a Textarea input. This is to support longer commit messages.
• Fixed textarea styling to match the existing text input fields (gray background, rounded border) — affects the existing Edit Message dialog too.

Testing

• npm run compile (lint + backend + frontend) passes clean.
• npm test: no regressions — failure count/suite list identical before and after.
• Verified the rebase mechanics end-to-end outside the extension (real git rebase -i runs) for both reword and combine.