store: rebuild ReadConversation around thread completion

[anish749]

Problem

ReadConversation had asymmetric filter logic: --since post-filtered by ts at the caller, while --date had no post-filter and interleaveThreads dumped every unmatched thread file at the end. Result: --date X for a day with no date file leaked unrelated thread data, and matched-thread replies on different days were treated inconsistently between modes.

Solution

Rebuild around one model. Both --date and --since are time windows. Pipeline is filter → thread-complete → dedupe → sort → interleave.

• A thread is the unit of context: any thread with ≥ 1 in-window member is kept whole, regardless of which member fell in window. Symmetric — parent-in-window pulls replies, reply-in-window pulls parent.
• Parents that live in both the date file and the thread file are deduped by ID. Thread atoms sort by parent ts (earliest reply ts for orphans); replies inside an atom sort by ts. Interleave is the last step.
• File discovery: date files are filtered by filename window via read.GlobFiles (filename is a directory-layout contract — type-safe). Thread files are listed whole and filtered at parse time using typed time.Time comparisons; pre-filtering by content patterns would couple to MsgLine.Ts's JSON serialization, which is fragile.
• The window/discovery code lives in internal/store/window.go — store layer owns it, calling internal/read for the rg primitives.
• --last=25 default for pigeon read lives in commands.RunRead (the UX layer). Store has no opinions; with no filter, it returns the full conversation. Hub callers (which pass Since explicitly) are unaffected.

Tests

• New: Date_NoFile_NoThreadActivity, Date_ParentInWindow_PullsLaterReplies, Date_ReplyInWindow_PullsOlderParent, Date_ThreadAtomSortedByParentTs, DedupsParentBetweenDateAndThreadFiles, NoFilter_ReturnsEverything.
• Updated: ThreadReply_OldParentInDateFile now asserts the old parent rides along — direct consequence of the new thread-completion model.
• Live-verified on real data: a day with no date file but thread files now returns empty (was 110 lines of leaked orphans); a day with a thread reply pulls the older parent + sibling replies into one atom.

Supersedes #358.