Rename sync to backfill, improve discoverability for AI agents

[kfastov]

Problem

The sync command is the main way to backfill chat history, but the name doesn't communicate this clearly. An AI agent trying to "download/index/export chat history" doesn't intuitively reach for sync. The actual workflow is:

1. sync jobs add --chat <id> --depth N — create a backfill job
2. sync --once — run backfill
3. sync jobs list — check progress

This took significant trial-and-error to discover. The channels sync --enable/--disable command adds to the confusion (is that the same sync? different?).

Proposal

1. Rename sync → backfill as the primary command name

   • Keep sync as an alias for backward compatibility (forks/existing users)
   • tgcli backfill --chat <id> --depth 5000 — single command to add job + run it

2. Simplify the common case:

   # Current: 2 steps
   tgcli sync jobs add --chat <id> --depth 5000
   tgcli sync --once
   
   # Proposed: 1 step
   tgcli backfill --chat <id> --depth 5000

3. Clarify relationship with channels sync — maybe rename to channels watch or channels follow to avoid the name collision.

Context

This is especially important for AI agent usage. Agents discover commands via --help and need clear, self-documenting command names. "backfill" immediately communicates intent; "sync" is ambiguous.