[TC-9] Worker main.rs entrypoints (replay/storage/agent) untested

[intendednull]

From #413.

Sev: low. Tier: unit (after extract). Obvious?: no.

main.rs of replay/storage/agent contain CLI parsing + config loading + service wiring. Zero tests.

Evidence:

$ rg '#\[(tokio::)?test|#\[cfg\(test' \
    crates/replay/src/main.rs \
    crates/storage/src/main.rs \
    crates/agent/src/main.rs
(empty)
$ wc -l crates/{replay,storage,agent}/src/main.rs
95 replay
91 storage
211 agent

CLI bugs surface only via just dev or production. Relay covered by #341 — this issue scopes the other workers.

Fix: Extract arg-parsing + config-resolution into each binary's lib.rs. Add unit tests on extracted fns.