feat: ownership oracles (externally computed reviewer requirements)

[zbedforrest]

Related PR(s)

#179 builds on this (inline ownership). Supersedes the seam explored in #45.

Summary / Background

Some ownership requirements depend on what changed inside a file, not which file changed (e.g. "telemetry event schema changes need data-platform review"). Path patterns can't express that, so such policies live in advisory bots that comment on PRs but enforce nothing.

This PR adds ownership oracles: an earlier workflow step computes reviewer requirements from the PR's content and hands them to codeowners-plus as a JSON file via the new oracle-files input. The rules are AND-merged into .codeowners-derived ownership through the existing MergeCodeOwners path, so review requesting, approval tracking, smart dismissal, and the status check apply unchanged.

Design properties:

• Add-only: oracle rules can add requirements but never remove or weaken .codeowners rules, so a tampered oracle file can at worst request extra reviews.
• Fail-closed: a missing or malformed oracle file (including an invalid glob pattern) fails the check rather than silently dropping reviews.
• A file matched by an oracle rule counts as owned for unowned-file reporting.

Code Changes

• pkg/oracle: JSON rule format, Parse/Load with strict validation, RuleSet.ToCodeOwners
• pkg/codeowners: NewFromFileOwners constructor for computed ownership
• internal/app: applyOracles merge step; main.go/action.yml: oracle-files input
• README: "Ownership Oracles" section; coverage badge regenerated
• Tests: oracle unit tests, app-level merge/error tests (multi-file, unowned interaction, case-insensitive approvals), splitOracleFiles

[gemini-code-assist]

Code Review

This pull request introduces 'Ownership Oracles' (computed ownership) to Codeowners Plus, allowing external tools to feed JSON-based reviewer requirements into the action. The changes include adding the oracle-files input, parsing and merging oracle rules with standard .codeowners requirements, and introducing the pkg/oracle package with accompanying tests. The review feedback focuses on improving robustness in pkg/oracle/oracle.go by validating glob patterns during parsing to fail-closed, and adding nil checks for warningWriter and overlays in Merge to prevent potential panics.