fix(remote-ssh): restore BYOK for Cursor 3.21+ SSH (#469)

[sunrisegm]

Summary

• Guides users to set cursorAgentHost.remoteInferenceRoute: "always" in Remote [SSH: host] (copyable UI; no auto User-settings write); local status labels stay local-only.
• Makes absolute-path Semble search run via Cursor-host Shell snapshot → local index → remote path rewrite; HTTP(S) Git repos stay on the existing path.
• Replaces WebFetch loopback/file URLs with conversation-private content_id paging (HTTP cache routes removed) so remote agents can read full results.

Closes #469

Test plan

• cargo test -p cursor-server --lib local_app::
• cargo test -p cursor-server --lib host_snapshot / search_provider
• cargo test -p cursor-server --lib search::cache::tests
• cargo test -p cursor-server --lib tool_call_dispatch::interaction::tests + tool_call_result::interaction::tests
• cargo test -p cursor-server --test tool_round
• npm run check in apps/desktop
• Live Remote SSH window: set remoteInferenceRoute=always, BYOK Agent stream, remote file/shell tools, Semble, WebFetch paging, Tab, commit generation (documented as unchecked in docs/remote-ssh.md; Cursor package verified as 3.22.12)

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