fix(skills): fix pi platform install and all-platform extension inclusion

[platinummonkey]

Summary

Fixes #559. pup skills install fails in two ways when pi is targeted:

1. pup skills install --name dd-apm pi gives a cryptic "no install target" error instead of explaining that pi only supports extensions.
2. pup skills install --name dd-apm all silently skips pi and reports a misleading platform count (7 platforms but nothing installs on pi).

Changes

Bug fix — better error for extension-only platforms (src/commands/skills.rs)

• When a skill/agent is requested for an extension-only platform like pi, emit a helpful error: "pi only supports extensions, not skills or agents. Available extension(s): dd-pup-pi"

New behavior — all always installs pi extension (src/commands/skills.rs)

• pup skills install all (with or without --name/--type filter) now also installs dd-pup-pi on pi, since all means "full experience on every platform"

Correctness fixes (src/skills.rs, src/commands/skills.rs)

• Platform count in success messages reflects actual platforms written to, not all selected platforms
• --dir multi-platform installs deduplicate writes by path (prevents redundant writes and inflated counts)
• Extension platform guard in install_paths and install_path is now unconditional — --dir can no longer bypass platform capability checks, preventing wrong attribution
• Agent-mode JSON "platforms" field reports only installed platforms

Code quality (src/skills.rs)

• Extracted PlatformSpec::is_extension_only() helper (was duplicated inline)
• format_as_agent_md delegates to format_as_skill_md (currently identical; doc comment notes when they diverge)
• Added impl PlatformSpec block for is_extension_only()

Testing

• 14 tests in commands/skills.rs including 6 new ones covering: original bug scenario, --type agent pi, --name dd-apm all, --type skill all, wrong-platform generic error, and dedup file count
• New tests in skills.rs: is_extension_only() unit tests, structural platform invariant, extension content-empty integrity check, pi+--dir returning None
• All 1233+ non-network tests pass; clippy clean under -D warnings

Related Issues

Closes #559

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