Phase 0: Harden plugin/lib/profile.sh with tests

[ericmmartin]

Goal

Bash-side profile reader used by all shell skills. Already stubbed in the Phase 0 spike; this issue is to harden it and add coverage.

Acceptance criteria

• packages/plugin/lib/profile.sh exposes profile_load, profile_get, and the documented CONDUCTOR_* env vars
• Validates required fields the same way the Zod schema does (name, project.number)
• Bats (or equivalent) test file: packages/plugin/lib/profile.test.sh
• Tests cover: valid profile loads, missing file errors clearly, missing required field errors clearly, default values applied for optional fields

Notes

Keep behavior in sync with shared/src/profile-schema.ts (#2). When that schema changes, this reader changes too — or the Zod schema should be the only source and the shell side reads via a Node helper.

Dependencies: #2

semver: patch