Archive staging-environments change; update local-dev spec

Staging environments are live and fully operational. Marks the remaining
verification tasks complete, syncs the delta spec to main specs, and
archives the change.

Also adds a "When to Use Local vs. Staging" reference table to the
local-dev-environment spec so the boundary between the two environments
is documented in one place.

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

openspec/specs/staging-environment/spec.md

@@ -0,0 +1,55 @@
+## Requirements
+
+### Requirement: Persistent staging instance
+A persistent staging instance SHALL run at `staging.trails.cool` (journal) and `planner.staging.trails.cool` (planner), auto-deploying from main on every push.
+
+#### Scenario: Deploy staging on main push
+- **WHEN** a commit is pushed to main that changes `apps/` or `packages/`
+- **THEN** the staging journal and planner containers are rebuilt and redeployed with the latest main
+
+#### Scenario: Staging uses isolated database
+- **WHEN** the staging instance is running
+- **THEN** it uses the `trails_staging` database, separate from the production `trails` database
+
+#### Scenario: Staging is accessible
+- **WHEN** a user navigates to `https://staging.trails.cool`
+- **THEN** they see the journal app served over HTTPS with a valid TLS certificate
+
+### Requirement: PR preview environments
+Ephemeral preview environments SHALL be created for each PR that changes app or package code.
+
+#### Scenario: Preview created on PR open
+- **WHEN** a PR is opened that changes files in `apps/` or `packages/`
+- **THEN** a preview environment is deployed at `pr-<number>.staging.trails.cool` within 5 minutes
+- **AND** a comment is posted on the PR with the preview URL
+
+#### Scenario: Preview updated on PR push
+- **WHEN** new commits are pushed to a PR branch with an active preview
+- **THEN** the preview containers are rebuilt and redeployed with the latest branch code
+
+#### Scenario: Preview torn down on PR close
+- **WHEN** a PR is merged or closed
+- **THEN** its preview containers are stopped and removed
+- **AND** its database (`trails_pr_<number>`) is dropped
+
+#### Scenario: Preview database isolation
+- **WHEN** a PR preview is running
+- **THEN** it uses a dedicated database `trails_pr_<number>` with schema applied via Drizzle Kit push
+
+### Requirement: PR preview cleanup
+A scheduled cleanup job SHALL remove orphaned preview resources from closed PRs.
+
+#### Scenario: Stale preview cleanup
+- **WHEN** the cleanup job runs
+- **THEN** any preview containers or databases belonging to closed/merged PRs are removed
+
+### Requirement: Resource limits
+Staging and preview containers SHALL have resource limits to protect production.
+
+#### Scenario: Memory limits enforced
+- **WHEN** a staging or preview container is running
+- **THEN** it has a memory limit of 256MB per app container
+
+#### Scenario: Concurrent preview limit
+- **WHEN** more than 3 PR previews are active
+- **THEN** the oldest preview is torn down before the new one is created