fix(session): prevent concurrent cross-process writes

[limityan]

Summary

• Ensure one native BitFun Runtime process writes a persisted Session at a time while allowing different Sessions in the same workspace to run independently.
• Keep SessionManager as the only Session lifecycle owner and reuse one OS file-lock implementation for Runtime ownership, Session writes, and serialized updates to the shared Session index.
• Make create/restore fail before publishing writable in-memory state, clean unfinished Session directories after cancellation, and keep transient or ephemeral Sessions out of persistence.

Behavior

Scenario	Result
Another process opens the same storage root and Session ID	Fails immediately with session_in_use; no polling or automatic takeover
Different Session IDs in the same workspace	Writers run independently; shared index updates are serialized
Path aliases resolve to the same storage root	Use the same writer identity
Same Session ID under different storage roots	File locks are independent; one SessionManager still keeps one active binding per Session ID
Create/restore fails, is cancelled, or times out	No writable Session is published; temporary lock and unfinished directory are released
Save/unload/delete fails	The loaded Session keeps its writer ownership
Unload/delete succeeds or the process exits	The OS lock is released; a stale lock file alone does not block recovery
List/view and non-persistent Sessions	Remain lock-free for Session writer ownership

Scope

• No new public SDK method, Server route, Shared Runtime protocol, background process, or CLI-to-SDK-Host dependency.
• Agent SDK, BitFun ACP, and Shared TUI retain a structured session_in_use classification. SDK Host exposes the existing retryable action_required; GUI, Embedded TUI, and Headless CLI continue to show an explicit text error without promising a structured field.
• Desktop acting as an ACP client manages external-agent Session records outside this native Runtime Session lifecycle and is intentionally not included.
• Durable behavior is documented in the existing runtime deployment architecture document; no PR-specific plan is committed.

Validation

Check	Result
cargo check --locked --workspace	Passed
cargo test --locked -p bitfun-core -p bitfun-desktop	Passed after the latest-main rebase: Core 1608 unit tests plus integration suites; Desktop 193 tests
bitfun-services-core	Passed: 86 unit tests plus all Session/index/write-lock contract suites
bitfun-acp / bitfun-cli	Passed: 109 / 377 tests
Agent Runtime, SDK Host, and SDK Host app suites	Passed
Core boundary checker and its 16 self-tests	Passed
Repository hygiene and git diff --check	Passed
Independent full-PR architecture and product reviews	P0/P1/P2: 0