feat: pi-first install system with extensions support

[bradleyrzeller]

What changed

The build/install pipeline now defaults to Pi and installs item-by-item instead of wiping destination directories. This means running make install overwrite won't clobber skills
or extensions installed by other tools (personal setups, other repos, etc).

New install flow:

  make install              # preview what would happen
  make install overwrite    # install, replacing conflicts
  make install wipe         # nuke destinations, then install

Configuration moved to install.toml:

  targets = ["pi"]                    # add "claude", "codex", "amp" as needed
  exclude = ["branch-quiz"]           # skip specific skills
  exclude-extensions = []             # skip specific extensions

  [overrides.some-skill]              # frontmatter overrides (moved from skill-overrides.toml)
  disable-model-invocation = true

Additions

• pi-extensions/ directory for team Pi extensions
• auto-session-name.ts — auto-names sessions after the first exchange
• Preview mode shows exactly what will be installed and flags conflicts (symlinks, existing directories)

Removed

• skill-overrides.toml — merged into install.toml under [overrides]
• Destination directory wiping as default behavior