Consolidate the reply-prefill readers into one shared helper

[jeremy]

Follow-up from #370 and #372, carded and approved as [CLI][TUI] Consolidate the reply-prefill readers into one helper.

The CLI (hey reply, hey compose --thread-id) and the TUI reply form each parsed the reply prefill (GET /entries/{id}/replies/new.json) — subject, acting sender, recipients — with its own reader. Reviewers on those PRs nudged four times toward a shared helper; declined there to keep each PR single-concern, and carded as a follow-up instead. This PR is that card.

What moved:

• New mail.ReplyPrefillFromServer (internal/mail/reply_prefill.go) is the one reader: it parses subject, acting sender, and recipients from HEY's prefill, and answers false — routing the caller to its local fallback — on a failed read or an empty recipient list, still carrying the subject and sender it did get.
• resolveThreadReply in internal/cmd/thread_reply.go now calls it; the package-local replyPrefill type and replyPrefillFromServer reader are gone. replyRecipients is now an alias of mail.ReplyRecipients, so the local fallback and the existing tests read unchanged.
• The TUI's loadReplyContext in internal/tui/compose.go drops its inline parsing block for the same call.

Pure refactor: no wire changes, and the existing reply tests in cmd and tui pass unchanged as the behavior lock, joined by a focused test for the helper itself. The auto_quoting invariant stays visibly intact and is now documented on the shared type: the prefill's quoted content is never carried or echoed back — HEY appends the quote at delivery.

One normalization note: the two readers disagreed only on degenerate prefill addresses (the CLI kept HEY's lists verbatim minus blanks; the TUI trimmed and case-deduped). The shared reader keeps HEY's computed lists verbatim, dropping blanks — identical output for anything HEY actually serves, and the tests lock that.

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Summary by cubic

Consolidates the CLI and TUI reply-prefill readers into one shared helper in internal/mail, so both reply flows parse HEY's prefill identically. Pure refactor — no wire changes; existing reply tests pass unchanged.

Refactors

• mail.ReplyPrefillFromServer reads subject, acting sender, and recipients, and returns false to trigger the caller's local fallback when the read fails or recipients are empty.
• internal/cmd/thread_reply.go and internal/tui/compose.go now call it; their package-local readers are deleted.
• The helper keeps HEY's recipient lists verbatim minus blanks; the TUI previously trimmed and case-deduped them, but output is identical for anything HEY actually serves.
• The prefill's quoted content is never carried; that invariant is now documented on the shared type.
• The reply-flow guide in AGENTS.md now points at the shared helper.

^{Written for commit af22ba0. Summary will update on new commits.}

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-01T21:07:37.171303Z	af22ba0	Manual request

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 8e84aacfe3

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[Copilot]

Pull request overview

Consolidates reply-prefill parsing into the shared internal/mail domain package for consistent CLI and TUI behavior.

Changes:

• Adds shared reply-prefill types and reader.
• Migrates CLI and TUI reply flows to the helper.
• Adds focused tests for success, fallback, and failure paths.

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Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
internal/mail/reply_prefill.go	Implements shared prefill parsing.
internal/mail/reply_prefill_test.go	Tests parsing and fallback behavior.
internal/cmd/thread_reply.go	Adopts the shared helper for CLI replies.
internal/tui/compose.go	Adopts the shared helper for TUI replies.

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[cubic-dev-ai]

All reported issues were addressed

<sub>Reply with feedback, questions, or to request a fix.

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[jeremy]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

Reviewed commit: af22ba0e42

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