docs: define story-number stability and source-tag examples for AC IDs

[singyichen]

Purpose

Close the two remaining Codex findings on the AC stable-ID standard introduced in #117: story-number stability and demonstrated source tags.

Reason

Two Codex findings were posted on #117 after it merged:

1. [High] The acceptance-criteria skill's Output Format examples did not demonstrate the (Source: AC-N.N) traceability tag the skill itself mandates, so generated checklists would likely omit it.
2. [Medium] AC-<story>.<seq> IDs embed the story number, but nothing forbade renumbering stories on insertion or priority reordering — which would silently break every published AC ID.

Result

Finding	Resolution
Missing Source tags in examples	AC-F01–F03 in the acceptance-criteria skill's Core Behavior examples now end with (Source: AC-1.1) / (Source: AC-1.2) / (Source: AC-1.3)
Story renumbering breaks AC IDs	spec-template v1.5.1 + spec-format skill now state: story numbers are identities, not positions — new stories take the next unused number, existing stories are never renumbered; display order may follow priority, but AC IDs follow the story number

Changed Files

File	Change
.specify/templates/spec-template.md	Story-number stability added to the AC ID guidance comment; changelog v1.5.1
.claude/skills/spec-format/SKILL.md	Same stability rule appended to the AC ID rules paragraph
.claude/skills/acceptance-criteria/SKILL.md	Core Behavior example criteria now demonstrate (Source: AC-N.N) tags

Test Plan

• Docs-only change — no code paths affected
• Template changelog row added (v1.5.1) and version header consistent
• Wording identical between spec-template comment and spec-format skill rule

Impact Scope

• Frontend
• Admin
• API
• Queue
• Other: SDD governance templates and skills

Related

#117

🤖 Generated with Claude Code

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[amazon-q-developer]

Documentation improvements are correctly implemented. The changes successfully address the two Codex findings by adding traceability source tags to examples and establishing story-number stability rules. The wording is consistent across spec-template and spec-format skill, and the changelog properly documents version 1.5.1.

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[gemini-code-assist]

Code Review

This pull request updates the specification templates and skill guidelines to clarify that User Story numbers are stable identifiers rather than positional indicators, ensuring that story numbers and their associated Acceptance Criteria (AC) IDs remain unchanged when display order is adjusted. Additionally, source mappings were added to the functional acceptance criteria in the skill documentation, and the template changelog was updated to version 1.5.1. The feedback recommends a minor phrasing improvement in the template instructions to clarify 'AC ID 的故事段' as 'AC ID 中的故事編號部分' for better precision.

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