feat: add 3 new error entries (concurrency-timing x1, triggers x1, yaml-syntax x1)

[htekdev]

New Error Entries — Strategy 6 Round 254 (Category Gap Fill)

Targeting the 3 categories with fewest entries: concurrency-timing (104), triggers (116), yaml-syntax (116).

ct-104 — Branch-Scoped Concurrency Group Allows Simultaneous Deployments to Same Environment

• File: errors/concurrency-timing/environment-deployment-branch-scoped-concurrency-cross-branch-collision.yml
• Severity: silent-failure
• Root cause: Concurrency groups keyed on github.ref/github.ref_name create separate slots per branch. Two branches deploying to the same environment have different groups and run simultaneously, causing deployment races. Fix: use job.environment as the concurrency key.

tr-118 — workflow_run requested Type Fires Before Upstream Executes

• File: errors/triggers/workflow-run-requested-type-fires-before-upstream-executes.yml
• Severity: silent-failure
• Root cause: The requested activity type fires when the upstream workflow is queued, before any steps run. Artifacts, outputs, and conclusion are unavailable. Also fires unnecessarily when the upstream run is later cancelled. Only appropriate for pure notification/audit workflows.

ys-120 — startsWith(), endsWith(), contains() Are Case-Insensitive

• File: errors/yaml-syntax/startswith-endswith-contains-case-insensitive-expression.yml
• Severity: silent-failure
• Root cause: These three expression functions are case-insensitive (unlike their equivalents in JS, Python, Go). Branch filters and deploy gates may match unintended casing variants silently. Fix: use == for exact case-sensitive matching, or toLower() to document intent explicitly.