[rosspeili]

Description

Fixes #86

Implements capability injection for multiple secrets and HTTP headers:

• Adds InjectTarget (ref, as, prefix) and inject: [...] list support on capabilities with 100% backward compatibility for single-secret declarations (secret: {ref, as}).
• Supports dot-path injection (e.g. headers.Authorization) and value prefixes (e.g. prefix: "Bearer ").
• Updates ToolHost.execute and SkillExecutor.execute protocols to forward headers.
• Membrane egress strips agent-supplied keys from both args and headers prior to broker injection, and scrubs all resolved secret values from tool.result.
• Enforces strict validation: rejects mutual declaration of secret + inject, forbidden secret keys, live tokens in refs, and duplicate target paths.

Type

• Bug fix
• Core framework — session, spine, constraints, identity, audit report
• CLI
• Integration — sequencer, hosts, membrane, observers, exporters
• Documentation
• Examples
• Packaging / CI

Checklist

• Linked issue (Fixes #86)
• pytest passes (254 passed)
• black aura tests — no diff
• flake8 aura tests — clean
• CHANGELOG.md updated when user-visible behavior changes

Test plan

• pytest — 254/254 tests passed across full suite.
• python scripts/aura_capability_stress_sim.py — 18/18 scenarios passed (added capability_multi_secret_dual and capability_header_authorization).
• python scripts/aura_host_stress_sim.py — 34/34 scenarios passed.
• python examples/14-capability-broker/main.py — 5/5 scenarios passed (added multi_inject).
• black --check aura tests integrations examples scripts — clean.
• flake8 aura tests integrations examples scripts — 0 errors.