Problem: contributors can't follow current spec lifecycle rules

[zolotokrylin]

Problem

Contributors following this repository's `CONTRIBUTING.md` for spec lifecycle
and Goal description format will conflict with the rules established in the
Wizard spec workflow. The guidelines currently say:

• Spec files are deleted once all sections graduate to `docs/`
• ETA is written inline as `ETA: ` in the Goal description

The Wizard spec workflow introduced a different set of rules:

• Spec files are never deleted — kept as frontmatter-only after graduation
• ETA lives in a `# Deadline` section, not inline
• `# Stakeholders Interview` is an allowed optional section
• Custom H1 sections are allowed for grouping issue references (e.g., `# Blocked by`)

Without updating `CONTRIBUTING.md`, contributors will follow the wrong format
and the GitHub App will flag their Goal descriptions as non-compliant.

Solution

Update `CONTRIBUTING.md` to reflect the spec format and lifecycle rules from:

• https://github.com/holdex/wizard/pull/1337