docs: add feature profiles and test plans for Git Mind product surface

[flyingrobots]

Summary

Add a serious feature-profile design-doc set for the Git Mind product surface after the git-warp v17 substrate upgrade.

Sponsor User

A technical lead, staff engineer, architect, or autonomous coding agent entering an unfamiliar repository and needing a trustworthy semantic map quickly.

Job To Be Done

When planning Git Mind features, help maintainers understand each product capability through IBM Design Thinking framing, user stories, requirements, and executable test strategy before implementation starts.

Scope

• One feature profile per major Git Mind capability or planned Hill feature
• User stories and requirements for each profile
• Fixture, golden, edge, known-failure, fuzz, stress, and regression test planning for each profile
• Index tying profiles to Hills, supporting lanes, and playback evidence

Exit Criteria

• New canonical design-doc set exists under docs/design
• Each feature profile has sponsor user, job, stories, requirements, and test plan
• Docs map points readers at the new design-doc set

[coderabbitai]

🔗 Related PRs

#176 - feat/next hex task [merged]
#301 - ci/docs: restore CodeQL and clarify documentation hierarchy [merged]
#302 - docs: adopt IBM Design Thinking governance [merged]
#308 - docs: define Hill 1 semantic bootstrap slice [merged]
#313 - docs: codify delivery cycle and fixture strategy [merged]

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_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

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