chore(openspec): add host-agent-single-instance-lock change artifacts

Proposal, design, spec, and tasks for the per-installation exclusive
instance lock. 15/16 tasks complete; only manual real-environment
verification (5.4) remains, with semantics covered by unit tests in
test_app.py and test_instance_lock.py.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

openspec/changes/host-agent-single-instance-lock/specs/host-agent-single-instance-lock/spec.md

@@ -0,0 +1,37 @@
+## ADDED Requirements
+
+### Requirement: Host Agent acquires an exclusive local instance lock before any control-plane side effect
+The Host Agent SHALL acquire an exclusive, non-blocking lock scoped to its configured `identity_path` state directory as the first action of application startup, before resolving host identity, enrolling any device, or contacting the control plane.
+
+#### Scenario: Lock acquired on normal startup
+- **WHEN** a Host Agent starts and no other process holds the lock for its `identity_path` state directory
+- **THEN** it acquires the lock and proceeds to identity resolution, device enrollment, and the heartbeat/claim loops as before
+
+#### Scenario: Duplicate instance is rejected before any network call
+- **WHEN** a second Host Agent process starts against the same `identity_path` state directory while a first process is still running
+- **THEN** the second process fails to acquire the lock and exits without resolving host identity, enrolling any device, or otherwise contacting the control plane
+
+### Requirement: Duplicate-instance rejection is immediate and clearly identified
+The Host Agent SHALL fail immediately, without waiting or retrying, when the instance lock is already held, and SHALL log an error that identifies the failure as a duplicate-instance conflict distinct from other startup failures.
+
+#### Scenario: Operator sees an actionable error
+- **WHEN** a duplicate Host Agent instance fails to acquire the lock
+- **THEN** the logged error names the lock file location and identifies the cause as another instance already running, rather than a generic or unrelated startup failure
+
+### Requirement: The instance lock releases without manual intervention after any process exit
+The Host Agent SHALL release its instance lock when its process exits for any reason, including graceful shutdown, an unhandled exception, or a forced kill, without requiring any manual cleanup step before a subsequent instance can start.
+
+#### Scenario: Lock is released after graceful shutdown
+- **WHEN** a running Host Agent instance completes its graceful shutdown sequence
+- **THEN** a new Host Agent instance started afterward against the same `identity_path` state directory successfully acquires the lock
+
+#### Scenario: Lock is released after an unclean process exit
+- **WHEN** a running Host Agent instance is terminated forcefully (e.g. killed) without running its shutdown sequence
+- **THEN** a new Host Agent instance started afterward against the same `identity_path` state directory successfully acquires the lock without any manual lock file cleanup
+
+### Requirement: Instance lock scope is per identity, not machine-global
+The instance lock SHALL be scoped to the Host Agent's configured `identity_path` state directory, such that two Host Agent processes configured with different `identity_path` values SHALL be able to run concurrently on the same machine.
+
+#### Scenario: Independent identities do not contend
+- **WHEN** two Host Agent processes start on the same machine with different `identity_path` state directories
+- **THEN** both acquire their respective locks and run concurrently without either being rejected