docs: add .squad directory explainer to first-session guide

[diberry]

What this PR does

Adds a 'What's inside .squad/?' section to the First Session guide, giving new users a quick reference for the files and directories Squad creates.

Changes

After the line where Squad creates the .squad/\ directory, a new subsection explains:

File/Directory	Purpose
\ eam.md\	Team roster — who's on the squad and their roles
\
outing.md\	Work routing rules — how tasks get assigned
\decisions.md\	Shared decision log — the team's institutional memory
\�gents/\	Agent charters and histories — each member's identity and learnings
\ceremonies.md\	Team ceremony definitions — standups, retros, design reviews
\casting/\	Name registry — persistent agent-to-name mappings

Also confirms that users can and should edit these files, and that they should be committed to version control.

Context

New users weren't sure what the .squad/\ directory contained or whether they should touch it. This explainer builds confidence right at the moment the directory is created.

Closes #289

Suggested reviewers: @diberry, @bradygaster

[diberry]

@bradygaster — PR is ready for review. Adds a quick-reference table to the first-session guide explaining what's inside .squad/ and confirming users can edit + commit those files.