fix(cli): flag, duration, and env-var parity with charon

[varex83agent]

Summary

Three CLI parity fixes against charon v1.7.1 (improvement plan area 06 — CLI flag / duration / env-var parity):

1. Go-equivalent duration parsing (T01)

CLI duration flags were parsed with humantime, which diverges from Go's time.ParseDuration: it accepts 1d/1w/1y/1M (Go rejects them) and rejects a leading + (Go accepts it).

• Adds parse_go_duration in crates/cli/src/duration.rs — a faithful port of Go's time.ParseDuration grammar (cmd/duration.go binds all charon duration flags through it): units ns/us/µs/μs/ms/s/m/h, multi-segment (1h30m), fractional (1.5h), leading +, the bare-"0" special case, and Go's 1<<63-1 ns overflow limit.
• Documented deviation: a leading - is rejected (Go accepts negative durations) because std::time::Duration is unsigned and every flag using this parser is a non-negative timeout/delay. Noted in a // PARITY: comment.
• Duration::FromStr keeps its bare-integer-nanoseconds branch (charon UnmarshalText strconv.ParseInt parity) and falls back to the new parser; the serde/JSON path is untouched.
• All value_parser = humantime::parse_duration sites repointed: test/mod.rs, test/validator.rs, test/peers.rs (×3), and test/beacon.rs (missed by the plan); the three dkg.rs flags pick it up via FromStr.
• humantime is dropped from the workspace (no remaining users).
• ~35 table-driven success/error tests added.

2. test validator missing --quiet guard (T02)

pluto alpha test validator --quiet without --output-json silently ran and produced no output at all. Adds the must_output_to_file_on_quiet guard as the first statement of run(), matching charon cmd/testvalidator.go PreRunE and the sibling mev/peers/infra/beacon subcommands, plus a test asserting the error path.

3. Relay command env vars PLUTO_* → CHARON_* (T03)

Charon binds every flag under the charon env prefix (cmd/cmd.go), so relay env vars are CHARON_HTTP_ADDRESS etc. The relay command was the only Pluto command using PLUTO_* (19 bindings). Renamed all 19 to CHARON_*; all 20 resulting env names in the file (incl. pre-existing CHARON_DATA_DIR) are unique, so no clap duplicate-env conflicts.

Out of scope (follow-ups per the plan): T04 alpha test all orchestration, T05 cosmetic help-text drifts.

Test plan

• cargo +nightly fmt --all --check ✅
• cargo clippy --workspace --all-targets --all-features -- -D warnings ✅
• cargo test --workspace --all-features — 52 test binaries, 0 failures ✅
• cargo deny check — fails on a pre-existing quick-xml advisory also present on main; unrelated (this PR's Cargo.lock diff only removes humantime)

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