Release session resources on partial-setup and error paths

[Soph]

Problem

Two leaks in the syncer session lifecycle:

1. newSession partial-setup failure. It opened the source transport (and started the memory-measurement ticker goroutine) before listing refs and setting up the target. If any later step failed it returned nil, err — so the caller had nothing to finish(), leaking the source conn (an SSH transport spawns a process), a half-open target conn, and the ticker goroutine.
2. finish() never stopped the measurement ticker. It was only stopped when a Result was built, so any early error return from runSync et al. leaked the goroutine even on a fully-constructed session.

Fix

• Add a success-guarded defer s.finish() in newSession, so a partial-setup failure releases everything; success = true is set only just before handing the session back (callers still defer s.finish() on success, so finish runs exactly once either way).
• Hand the target conn to s.target as soon as it's opened, so cleanup can close it even if a target ref-listing step fails.
• Have finish() also invoke the sync.Once-guarded measurementDone, stopping the ticker on every path without disturbing the happy-path measurement value.

Tests

TestStartMeasurementStopsGoroutineAndIsIdempotent confirms the closure stops its goroutine and is safe to call twice (the property finish() now relies on); TestStartMeasurementDisabledIsInert covers the disabled case. Full go test ./... passes.

🤖 Generated with Claude Code

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Note

Medium Risk
Touches core session lifecycle and connection cleanup on failure paths; behavior change is limited to resource teardown but affects every sync entry point that uses newSession.

Overview
Fixes resource leaks when newSession fails mid-setup or when a sync exits before building a Result.

newSession now uses a success-guarded defer s.finish() so partial failures still tear down the source/target transports (including SSH subprocesses) and the memory-measurement ticker started at the beginning of setup. The target connection is attached to s.target as soon as it is opened so later ref-listing errors still get closed.

finish() also invokes the sync.Once-guarded measurementDone closure, stopping the measurement goroutine on error paths without changing the value already captured on the happy path.

New tests in measurement_test.go assert that startMeasurement stops its goroutine and remains safe when called twice (the behavior finish() now depends on).

^{Reviewed by Cursor Bugbot for commit f81cf1c. Configure here.}