hey reply: override recipients (--to/--cc) without losing the thread

Start by tracing the hey reply entry point and compare it with hey compose --thread-id and hey thread read --json, which the issue identifies as related paths. Done means reply accepts recipient overrides and --replace-recipients, supports --dry-run --json without sending, and preserves the thread id, topic, References, and In-Reply-To.

Context

Agents often need to reply in-thread to an address that is not what HEY would pick from the last message — especially with alias / reverse-alias forwards (plus-addressing), ticket systems, and “noreply” envelopes where the human address lives elsewhere.

hey compose --thread-id can approximate this but is easy to get wrong and still isn’t a first-class reply path. After v1.6.0, agents can see the right address (received_via, recipients in hey thread read --json) but still can’t target it on hey reply.

Related: #446 (broader agent ask; this issue splits out the highest-priority remaining item), #431 (envelope visibility — addressed in 1.6.0 via received_via).

No account-specific details below — capability only.

Ask

Add recipient overrides on reply that keep threading:

hey reply <thread-id> --to person@example.com -m "..."
hey reply <thread-id> --to a@x.com --cc b@y.com -m "..."
hey reply <thread-id> --to person@example.com --replace-recipients -m "..."
hey reply <thread-id> --to person@example.com --dry-run --json

Behavior

1. --to / --cc / --bcc (repeatable or comma-separated — either is fine if documented)
2. --replace-recipients — when set, overrides replace HEY’s resolved reply recipients instead of merging (recommended default for agent use: replace To at least)
3. --dry-run — print resolved From / To / Cc / Bcc / thread id as JSON; do not send
4. Must remain a reply (same topic / References / In-Reply-To), not a new compose

Out of scope

• Connecting SMTP send-as addresses (see #454)
• Bulk trash / sweep filters / body hydration (covered or tracked elsewhere)
• Auto-detecting a “suggested reply-to” from body text (nice later; not required here)

Why

1.6.0 gave agents the data (received_via). This gives them the action. Together they remove the main footgun in automated alias-forwarded support/seller mail.

Environment

• hey 1.6.0
• Primary interface: --json from agents/automation