Add pull request template

[lukidoescode]

Description

Adds .github/pull_request_template.md so GitHub pre-fills new PR descriptions with a structured template tailored to a Kubernetes operator project. Replaces ad-hoc PR shapes (and the generic ## Summary / ## Test plan default) with explicit prompts for the dimensions that matter here.

Template sections: linked issues (top bullets, matching the existing merged-PR convention), Description, Decisions Made, CRD / API Compatibility, Risks and Mitigations, Rollback Strategy, Verification. Helper prompts live in HTML comments so the rendered PR stays clean. No checkbox theater, no duplication of changeset-owned release notes.

Decisions Made

• HTML-comment hints rather than visible italic prompts — keeps rendered PRs clean.
• Linked issues as top-of-PR bullets, not under their own heading — matches Implement lldap-operator-traits crate with async trait abstractions #30, Define LldapUser/Group/Membership/AttributeSchema CRDs #31 convention.
• Kept N/A policy explicit in the leading comment so reviewers can distinguish "considered and skipped" from "forgot".
• Omitted checkbox blocks, "Type of change" radios, and a "Release Notes" section (changesets own that).

CRD / API Compatibility

N/A — no code, CRD, RBAC, or webhook changes.

Risks and Mitigations

• Risk: template is too long and contributors ignore it. Mitigation: every section earns its place; HTML comments mean the rendered output is short.

Rollback Strategy

Delete .github/pull_request_template.md. No state, no migration.

Verification

• Opened the file locally to confirm it renders correctly.
• After merge: open a draft PR on any branch and confirm GitHub pre-fills the description with the template body.
• No changeset required per CLAUDE.md (CI/docs-only change does not affect crate behavior).