Reply to explicit recipients without losing the thread

[robzolkos]

What changed

• add repeatable and comma-separated hey reply --to, --cc, and --bcc recipient overrides
• merge overrides into HEY's reply prefill by default, moving an explicitly named address to its requested line without duplicates
• add --replace-recipients for a fully explicit envelope
• add a read-only --dry-run that reports the resolved account, thread, entry, subject, sender, and final recipients without reading a body, uploading attachments, or sending
• let explicit recipients make a reply target addressable when HEY cannot resolve recipients itself
• apply the same final envelope when saving a reply draft

Replies continue through the existing entry reply endpoint, so they remain in the original thread.

Verification

• make test
• make lint
• HEY_SMOKE_STRICT=1 go test -run '^TestReplyRecipientOverrides$' -count=1 -v in tests/smoke
• built binary help and baseline CLI reproduction

Closes #463

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Summary by cubic

Adds hey reply recipient overrides (--to, --cc, --bcc, repeatable and comma-separated) so replies can address explicit recipients without leaving the thread. Overrides merge into HEY's prefill, moving a named address to its requested line; --replace-recipients uses only the explicit list. A read-only --dry-run reports the resolved account, thread, entry, subject, sender, and final recipients without sending. Explicit recipients now let replies go out even when HEY cannot resolve recipients itself, and the resolved envelope applies to drafts as well.

• Dry runs and --replace-recipients sends never read the original message body.
• When HEY's prefill is unavailable, a dry run refuses to guess the original recipients and suggests --replace-recipients with explicit addresses.

^{Written for commit 77f2039. Summary will update on new commits.}

[Copilot]

Copilot review overview

🟡 Changes recommended

Dry runs and replacement envelopes can still fetch—and fail on—the original message body before explicit recipients are applied.

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Review effort: Balanced
Findings: 1

Open (1)

• Apply recipient overrides before body-dependent thread fallback · New

What changed in this PR

Adds configurable reply recipients and a read-only envelope preview while preserving in-thread delivery.

Changes:

• Adds merge/replace recipient overrides.
• Adds --dry-run envelope previews.
• Updates tests, documentation, skill guidance, and CLI surface.

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File	Description
.surface	Records new reply flags.
docs/​cli.md	Documents overrides and dry runs.
internal/​cmd/​compose.go	Rejects unaddressed thread replies.
internal/​cmd/​compose_test.go	Tests compose rejection behavior.
internal/​cmd/​reply.go	Implements flags and preview output.
internal/​cmd/​thread_reply.go	Resolves senders and merges recipients.
internal/​cmd/​thread_reply_test.go	Tests reply envelopes and previews.
internal/​mail/​reply_prefill.go	Exposes the resolved reply sender.
internal/​mail/​reply_prefill_test.go	Tests sender extraction.
skills/​hey/​SKILL.md	Updates agent reply guidance.
tests/​smoke/​threads_test.go	Smoke-tests override delivery.

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[cursor]

Approved. Cursor Bugbot was not running; Cursor Security Agent passed with no findings that need human review. No reviewers were assigned.

  

_{Sent by Cursor Approval Agent: Pull Request Approver}

[robzolkos]

Fixed - dry runs and replacement envelopes no longer fetch the original message. Tests cover failed and oversized body reads, and non-replacement previews refuse to guess unresolved recipients.