MAI-58: honour --dry-run in commits-only mode (no add/commit/push)

[vsmash]

Summary

In commits-only mode (-c / -ac), --dry-run (-d) was silently ignored: it performed a REAL git add + git commit, and with -ac a REAL git push. dryRun was parsed and threaded only into version management, never into the commit phase.

Fix

• Thread dryRun from runMaiassPipeline → handleCommitWorkflow → commitThis → handleStagedCommit (lib/maiass-pipeline.js, lib/commit.js).
• Guard every real git mutation in the commit phase (git add -A, git commit -F, git push --set-upstream) so that when dryRun is true the command is NOT executed and a clear preview line is printed instead.
• In a forced dry run nothing is actually staged, so handleStagedCommit previews the working-tree diff (git diff HEAD --name-status) and labels it "Changes that would be committed:".
• Commits-only completion line now reflects the dry run ("Commits-only mode dry run completed (no changes made)").
• Preview lines use console.log(colors.BBlue(...)) to match the existing version-management dry-run UX (always visible, not filtered by brief verbosity).

Preview wording

• Dry run - would stage all changes (git add -A)
• Dry run - would commit with message: "<first line>"
• Dry run - would push to origin <branch> (git push --set-upstream origin <branch>)

Verification (isolated sandbox: file:// bare origin, throwaway identity)

• -ac --dry-run (AI off): HEAD unchanged, index empty, origin branch not created; all three previews printed.
• -ac (no --dry-run): still really commits and pushes — happy path intact.

Tests

• Added test/unit/commit-dryrun.test.js: builds a real temp repo with a bare origin, asserts dry-run does NOT move HEAD / stage / push, and that a normal run does commit + push.
• npm run test:unit (60 tests) and npm test (10 e2e) both pass.

Parity note

This changes the commit-phase behaviour only on the --dry-run path; the normal commit/push flow is unchanged. Flagging for bash-cli-specialist to confirm bashmaiass honours --dry-run in commits-only mode equivalently.

Jira: https://velvary.atlassian.net/browse/MAI-58