fix(session): surface cross-process writer conflicts

[limityan]

Summary

• present the existing cross-process session writer conflict consistently in Embedded/Shared TUI, Desktop, Peer Host, and headless output
• keep session history readable while a different BitFun instance owns writes, with an explicit one-shot Retry action in the GUI
• preserve newer sends, turns, and composer edits when an older request or retry completes late
• document the existing ownership boundary without changing ACP, SDK, Runtime IPC, or session storage contracts

Product boundary

• no automatic waiting, retry loop, writer takeover, or controller transfer
• no new session ownership protocol or error envelope
• no change to read-only session history
• no ACP or Agent SDK behavior change

Verification

Area	Result
Workspace compile	cargo check --workspace passed
Desktop	cargo test -p bitfun-desktop - 194 passed
Session writer lock	cargo test -p bitfun-services-core --test session_write_lock_contracts - 10 passed
CLI conflict contracts	Embedded/Shared diagnostics, JSON, stream-json, and Runtime IPC conflict tests passed
Headless CLI integration	cargo test -p bitfun-cli --test exec_cli_contracts - 15 passed
Web focused paths	78 passed across message/session/draft/error/notification/Peer tests
Web type and lint	type-check:web and lint:web passed
i18n	contract 37 passed; audit passed with 0 warnings
Architecture/repository guards	core boundaries and repository hygiene passed

Two current-main Windows baseline issues remain outside this diff:

• full CLI: 412 passed and 4 tests from feat(dispatch): add detached SSH task execution (#1850) failed; the first failing test was reproduced by itself on an independent clean gcwing/main@6e3d0ef59 worktree with the same os error 5
• full Web: 2346 passed and the unchanged RemoteConnectDialog.contract.test.ts failed because it hard-codes LF while the Windows checkout source uses CRLF

This PR does not modify either failing area.