fix(cli): make rtk stats a real alias of gain (#3263)

[NanoRisk6]

What Problem This Solves

rtk stats exited 127 with only [rtk: No such file or directory (os error 2)] and no table. Users expected savings stats (like rtk gain); instead the CLI fell through parse-fallback and tried to spawn a PATH binary named stats. Fixes #3263.

Why This Change Was Made

1. Add clap visible_alias = "stats" on gain
2. Add stats to RTK_META_COMMANDS so bad flags never fall through to raw spawn
3. Regression test: rtk stats --daily parses as Gain { daily: true }

User Impact

rtk stats (and flags like --daily) works as token-savings summary — same as gain. No more opaque 127 when ccusage/npx are unrelated.

Evidence

cargo test --bin rtk stats_alias
# test_stats_alias_routes_to_gain ... ok

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