feat(install): add native OMP support

[ousamabenyounes]

Fixes #743

Summary

• Add omp provider detection and native installer routing.
• Prepare a managed OMP plugin package at ~/.omp/caveman-plugin with package.json#omp, extension entry, skills, commands, agents, and activation rule, then delegate lifecycle to omp plugin install and omp plugin uninstall.
• Restore any previous prepared package if omp plugin install fails.
• Update installer/security docs for the new user-facing OMP support.

OMP API Verification

Verified against upstream Oh My Pi docs/source before coding:

• local plugin installs use omp plugin install <path>;
• package plugin metadata is declared through package.json#omp;
• installed plugin roots expose sibling skills/, commands/, agents/, and rules/;
• extension entrypoints can register session_start and update UI status through the plugin context.

Test Verification

RED, new OMP tests before installer support:

# Subtest: omp fresh install prepares plugin package and invokes OMP plugin install
not ok 1
error: |-
  argv error: error: unknown agent: omp
    see 'caveman --list' for valid ids
# Subtest: omp uninstall uses plugin lifecycle and removes prepared package
not ok 2
1..2
# tests 2
# pass 0
# fail 2

GREEN, focused OMP tests:

node --test tests/installer/omp.test.mjs
1..3
# tests 3
# pass 3
# fail 0

Local CI baseline from upstream/main:

./run-ci.sh
1..112
# tests 112
# pass 112
# fail 0

Local CI final on fix/issue-743:

./run-ci.sh
1..115
# tests 115
# pass 115
# fail 0