feat(gmail): add forward command with attachment support

[brettdavies]

Summary

Add gog gmail forward <messageId> command that forwards an email with all original attachments in a single command. Closes #166.

New flags: --to, --cc, --bcc, --subject, --body, --body-file, --from, --attach (repeatable), --no-attachments

Scope

• New command: gmail forward in the "Write" group (alias: fwd)
• Forwards original attachments by default; --no-attachments to strip them
• --attach adds additional local files alongside forwarded attachments
• Prepends "Fwd: " to subject (case-insensitive skip if already prefixed); --subject to override
• Optional body preface via --body / --body-file, inserted before forwarded header block
• --from for send-as alias support via existing resolveComposeFrom()
• --json / --plain output consistent with gmail send
• Thread preservation: sets ThreadId on the sent message so the forward stays in the sender's thread, matching Gmail web UI behavior
• Reuses existing helpers (buildRFC822, resolveComposeFrom, collectAttachments, fetchAttachmentBytes, writeSendResults) — no existing files modified beyond 1-line command registration in gmail.go

Design notes

• Reviewed PR feat(gmail): add forward command with tests and helpers #167 by @salmonumbrella which informed the design. Key differences from that PR:
  • Calls resolveComposeFrom() directly instead of creating a new resolveSendFrom() — avoids modifying gmail_send.go
  • Adds --attach for local files and --no-attachments to strip originals
  • Preserves thread (sets ThreadId on sent message)
  • Focused scope: no unrelated docs/drive/time changes
  • Recipients via flags (--to) matching gmail send pattern, not positional args

Testing

• Unit tests (gmail_forward_test.go): 30 subtests covering forwardSubject, forwardHeaderPlain/HTML, buildForwardBodies, plainToHTML, joinForwardSections
• Integration tests (execute_gmail_forward_test.go): 7 tests via Execute() with httptest mock servers:
  1. Default subject + attachments
  2. HTML-only message (plain text fallback)
  3. Custom subject override
  4. CC/BCC recipients
  5. --no-attachments strips originals
  6. --attach adds local files alongside originals
  7. --no-attachments + --attach replaces originals with locals
• All existing tests pass (go test ./...)

Known limitations (v1)

• Inline images (CID-referenced parts) become regular attachments
• No --body-html for rich-text preface (plain text auto-escaped for HTML variant)

Files

Created:

• internal/cmd/gmail_forward.go (264 lines) — command + helpers
• internal/cmd/gmail_forward_test.go (424 lines) — unit tests
• internal/cmd/execute_gmail_forward_test.go (763 lines) — integration tests

Modified:

• internal/cmd/gmail.go (+1 line) — command registration

[brettdavies]

@steipete do you mind reviewing this PR when you have the chance? Having the capability to forward will unlock a crucial automation step in a couple of key workflows. Thank you