fix(core): add flock to serialize session file read/write

[chenhg5]

Summary

• Add exclusive advisory file locking via flock(2) to SessionManager.load() and SessionManager.saveLocked()
• Lock file: {storePath}.lock - acquired exclusively before read/write
• This prevents race conditions when multiple projects or goroutines concurrently access the same session state file

Fixes #324.

Test plan

• go build ./... passes
• go vet ./... passes
• go test ./... passes

Closes #324

[chenhg5]

Note: CI lint check failed with the following errors:

core/session.go:450:13: Error return value of `unix.Flock` is not checked (errcheck)
core/session.go:451:19: Error return value of `unix.Flock` is not checked (errcheck)
core/session.go:498:13: Error return value of `unix.Flock` is not checked (errcheck)

Please consider handling the error return values from unix.Flock calls before merging.