Introduce configuration initialization

[Adjanour]

Summary

Build the sess config init workflow described in Phase 4 so users can set global and per-repository defaults interactively.

Motivation

• README.md Phase 4 and docs/technical-spec.md specify configuration management as a key milestone.
• Centralized configuration reduces ceremony for branch prefixes, base branch selection, and PR templates.

Scope

• Add sess config init that writes to ~/.sess-cli/config.json and optional repo-level overrides.
• Support settings for organization, default base branch, branch prefixes, PR template path, and hooks.
• Validate inputs and provide sensible defaults matching current behavior.
• Ensure other commands consume the new configuration (e.g., start/end workflows).
• Document the hierarchy (global vs. per-repo) and persistence locations.

Out of Scope

• GUI configuration tools.
• Remote/shared configuration.

Acceptance Criteria

• Running the command produces config files matching the documented schema.
• Commands respect overrides without requiring manual flag passing.
• Documentation updates explain usage and precedence.

References

• README.md (Phase 4 roadmap)
• docs/technical-spec.md
• docs/DEVELOPMENT.md