Archiving a session should relocate it out of the startup scan path

[ACGNnsj]

Summary

"Archive session" currently only sets a marker: unarchiving removes the marker, the save location never changes, and the row is merely hidden from the list (there is a filter to view archived sessions). The session files stay in sessions/<workspace>/, so startup enumeration and eager projection still read them. Archiving cleans up the visible list but provides zero startup-time or memory relief.

Motivation

Real-world installation (dsh-desktop 0.2.2-rc.1, win32 x64, 64 GB RAM):

• sessions/: 10,502 files, ~1.26 GB (session.v4.jsonl.zstd, one per session) — venera 6,252 files/434 MB, bot-workspace 1,925 files/539 MB, consoles 908 files/109 MB, dsh 674 files/82 MB; largest single session log 38 MB compressed.
• After a cold start the sidebar workspace tree takes very long to expand, and every unopened session shows an "unnamed" title (titles are generated lazily on open; most of these sessions are subagent-spawned with no user first message).
• 6 host OOM crashes in 2 days: exit 134, FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - JavaScript heap out of memory, heap dying at ~2.4–2.9 GB after last-resort GC, 26 min–2 h after start. One stack shows the fatal allocation landing inside TurboFan v8::internal::compiler::CompilationDependencies while the heap is full — i.e. the bloated retained heap (dominated by session projection) leaves no headroom for background compilation.
• Related: [Bug] Eager projection-cache load dominates `dsh web` startup: 969 records / 209 MB → 5.3 s of validation (26%) #11608 (projection cache dominates startup), Host OOM crash loop at startup: cold session projection eagerly full-reads every session log #10182 (OOM in startup session projection).

Proposed Solution

When a session is archived, relocate its directory to an archive location that is excluded from the startup scan and from eager projection; move it back on unarchive. (Alternative, weaker: exclude archived session ids from eager projection and from cold enumeration, while keeping files in place.)

Acceptance Criteria

• Archived sessions are not read during startup (no listing IO, no projection memory).
• Unarchiving restores the session to its workspace with history intact.
• A restart no longer re-scans archived content.