fix(web): keep synthetic session keys verbatim so marks survive persistence

[onychen]

Problem

Fixes #351.

When the current workspace is removed while its active session has not yet persisted (no session file yet), removeWorkspace keys the session into ungroupedSessions with the synthetic current:<sessionId> marker — but passes it through resolve() first, persisting <cwd>/current:<uuid>. The polluted key only matches the projection while both sides resolve identically inside the same process. Once the session gains a real file (first message), the mark is lost forever: the session disappears from every sidebar group, and /api/sessions/select is rejected by requireSession with "Workspace is not available". archiveSession applies the same pattern and loses archived the same way.

Value

• The workspace-delete confirmation promise ("Its conversations will move to Ungrouped") now holds for the active, not-yet-persisted conversation — today it breaks on the very first message.
• Archiving an unpersisted session keeps the archive mark across persistence.
• No more fake <cwd>\current:<uuid> entries accumulating in workspace-state.json / archived-sessions.json.

Approach

Follows the minimal fix proposed in #351:

• Synthetic current:<sessionId> keys now round-trip verbatim: a shared sessionKey() helper (with isSyntheticSessionKey) is used on write (removeWorkspace, archiveSession) and on load (ensureWorkspaceStateLoaded, ensureArchivesLoaded); real paths keep being resolved as before.
• Projection lookups go through a new hasSessionMark() helper that matches a mark by either the resolved file path (marks recorded after persistence) or the synthetic current:<id> key (marks recorded before it), so marks survive the session gaining a real file without any migration.
• Legacy polluted entries (<cwd>\current:<uuid>) resolve idempotently and simply never match anything — inert, left in place per bug(web): removing the current workspace hides its unpersisted active session instead of moving it to Ungrouped #351.

Validation

• npm run format:check, npm run lint, npm run typecheck — pass.
• node --test --experimental-strip-types tests/web/pi-adapter.test.ts — 15/15 pass, including 2 new regression tests that reproduce the full unpersisted → persisted transition for both the ungrouped and archive flows.
• Verified the new tests fail against the unfixed adapter (4 failing tests with the fix stashed), so they genuinely lock the bug.
• tests/web pure-logic suites (protocol, http-dispatcher, observer-registry, terminal-status, web-host-lease): 38 pass / 0 fail / 2 platform skips; app-render.test.ts: 21/21.
• Full node scripts/run-tests.mjs on Windows hits the pre-existing background-terminal kill-test flakes/hang tracked in test: eliminate Windows background-terminal process-test flakes in the full suite #304 (process-management domain, unrelated to this change); not fully runnable on this machine.

Impact

• User-visible: removing the current workspace keeps its active unpersisted conversation in Ungrouped after it persists, and it remains selectable; archiving an unpersisted session keeps the mark after persistence.
• Persisted data: new marks are stored verbatim as current:<id>; legacy polluted entries remain readable and inert (no migration, per bug(web): removing the current workspace hides its unpersisted active session instead of moving it to Ungrouped #351). No schema change.
• Model-visible context / runtime lifecycle: none.
• Compatibility: marks written by the old code (polluted keys) were already non-matching orphans, so existing state sees no regression.

[onychen]

改用中文描述重新提交，新 PR 见后续评论。