Implement `cmd/exit delete`

[varex83]

Scope

Port charon/cmd/exit_delete.go (~140 LOC) — the pluto exit delete subcommand that removes a partial exit from the obol-api store.

References

• Go: https://github.com/ObolNetwork/charon/blob/main/cmd/exit_delete.go
• Tests: cmd/exit_delete_internal_test.go (~179 LOC)

Acceptance

• Flag/help parity with charon
• Tests ported and green
• cargo clippy --workspace --all-targets --all-features -- -D warnings clean