📝 docs(specs): define standard spec format and conventions

[Copilot]

Specs lacked a canonical format, had two divergent templates, and no explicit rules on required sections, level of detail, or AI readability — making them unreliable inputs for agents and inconsistent across the repo.

Changes

specs/_template.md — single canonical template

• Added Inputs and Outputs tables (type, required, validation rules)
• Expanded Behavior into three mandatory subsections: Happy Path, Edge Cases & Alternate Flows, Error Handling
• Added Constraints section (performance, security, accessibility, platform, dependencies)

specs/README.md — authoritative conventions reference

• Required vs. Optional Sections table — explicit per-section status and purpose
• Level of Detail guidelines — concrete rules for behavior descriptions, acceptance criteria, inputs/outputs, and constraints; bans vague language like "works correctly"
• AI Readability guidelines — structured lists over prose, consistent terminology, explicit links, one criterion per bullet
• Tightened File Naming to note _-prefixed files are reserved for meta/tooling

specs/templates/feature.spec.md — removed

Redundant alternate template; best content absorbed into _template.md. Having two templates was the root cause of inconsistency.

CONTRIBUTING.md — added Feature Specifications section

Five-step summary of the spec-driven workflow with an explicit callout that incomplete or vague specs produce incorrect AI implementations.

[Copilot]

@szmyty Unfortunately, I hit an error while trying to use the custom Copilot setup steps configured for this repository and had to close this PR. The error I am seeing is:

no `copilot-setup-steps` job found in your `copilot-setup-steps.yml` workflow file. Please ensure you have a single job named `copilot-setup-steps`. For more details, see https://gh.io/copilot/actions-setup-steps

Once you or someone with the necessary access fixes the problem, please unassign and then reassign issue #0 to me and I'll retry. Thanks!

[devactivity-app]

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