fix(install): remove bundled goal integration

[zhanfeng-alpha]

Why

Remove @narumitw/pi-goal from pi-stack rather than installing and configuring a session goal runner.

Scope

• Remove the package requirement, manifest validation, pi-goal.json creation, and legacy goal-prompt migration.
• Remove goal-specific instructions and package documentation.
• Prune old npm registrations by package identity, including pinned and object-form entries.
• Replace positive goal integration checks with package and generated-config absence assertions.

Blast Radius

This is an independent main-based PR. Existing user-owned config files and prompts are preserved. The live host and current session are unchanged. No replacement goal runner is introduced. Leave this PR open; do not merge.

Verification

Parent-inline in /home/alphalab/.pi-stack-worktrees/remove-goal. No child agents launched.

• check-overlay.sh passed cold and repeat installs with an exact package-request list excluding the goal package, no generated pi-goal.json, and removal of old registrations.
• check-update-pstack.sh, check-conform-skills.sh, and check-subagents.sh with smoke disabled passed.
• Shell syntax and Git whitespace checks passed.
• Remaining package references are limited to removal logic and absence/preservation tests.