Pipeline docs name the discovery stage

[sameera]

As a Nexus user reading the docs, I want the pipeline documentation to name the discovery stage and when it applies, so that I know to run /nxs.discover before /nxs.epic for a foggy initiative.

Acceptance Criteria

• Given the repository docs, when they describe the pipeline, then README.md, how-to-nexus.md, and the project CLAUDE.md all show setup → (discover when foggy) → epic → decision-record → analyze → close → distill.
• Given the discovery stage's doc entry, then it states the stage's durable contract: discovery is a multi-session loop, its unit is the decision ticket, its output is functional goals that /nxs.epic files, and a discovery can be shared by ordinary git operations. The collaboration property is part of the contract, because a reader needs it in order to know they can hand a discovery to someone else.
• Given the same doc entry, then where the store physically lives is stated in a single sentence per document, and that sentence is explicitly marked as first-iteration. The contract outlives the storage choice, so confining the storage sentence keeps a later migration to an issue-backed store a one-line documentation edit.
• Given the docs, then they distinguish oversized (big but clear, /nxs.epic's decomposition path) from underspecified (foggy, /nxs.discover).
• Given the docs, then they state that discovery writes nothing to GitHub, and that the issues appear when /nxs.epic consumes the finished discovery.