Multiple accounts via suffixed env vars

[thepeak99]

What

Adds multiple-account support to mail-cli, staying fully env-only and stateless. Each account is a set of MAILCLI_* env vars sharing a suffix (_1, _2, …); a single run targets one account.

mail-cli list                 # default account = bare MAILCLI_* vars (unchanged)
mail-cli --account 1 list     # reads MAILCLI_*_1
mail-cli accounts             # discover configured accounts (never prints passwords)

Design

• Selection precedence: --account <TOKEN> (global flag) > MAILCLI_ACCOUNT env > none.
• No --account → the default account = the bare, un-suffixed vars → zero breakage for existing setups.
• Token is numeric by convention but any non-empty string works (one code path); empty token rejected loudly.
• Strict, no fallback: account N is read only from *_N; a miss fails naming the exact var (e.g. required environment variable MAILCLI_PASSWORD_2 is not set).
• Config values stay env-only; the runtime choice of account is a CLI flag — consistent with the repo's boundary rule. All env reading stays in config.rs.

Changes

• config.rs — Env { suffix } resolver (name-building at the boundary) + from_env(&Env); scan_accounts()/AccountInfo for discovery (emits has_password presence only, never the value).
• main.rs — global --account flag; new accounts subcommand; &Env threaded into connecting commands. install-skill untouched.
• output.rs — accounts_human table (ok/incomplete, no secrets).
• Docs — README "Multiple accounts" section + accounts; AGENTS.md security rule.
• Skill — SKILL.md + references/reference.md (AccountInfo schema) + 2 new evals; verified install-skill --force output is byte-identical to skill/.
• ACCOUNTS-PLAN.md — locked design doc (mirrors the repo's plan-doc convention).

Tests / CI

• +11 config.rs unit tests: name resolution, empty-token reject, suffixed-error naming, scanner (default+numeric, incomplete-no-leak, sort order, empty-value skip).
• 45 tests pass (16 lib + 26 bin + 3 integration).
• Ran the exact CI checks locally: cargo fmt --all -- --check, cargo clippy --all-targets --locked -- -D warnings, cargo test --locked — all green.

Co-authored by PeakBot!