feat: enrich generic HTTP post actions

[shivros]

Summary

Implements COD-466's approved generic-only http_post enrichment:

• optional deterministic body_template substitution over normalized event fields and nested metadata
• optional request headers, with text/plain default for templated bodies
• unchanged JSON event forwarding when no template is configured
• generic webhook documentation/example; no provider-specific action variants

Verification

cargo build --all-targets                         PASS
cargo test --all-targets (16 tests)               PASS
cargo clippy --all-targets -- -D warnings         PASS
cargo fmt --all -- --check                        PASS
git diff --check                                  PASS

Closes COD-466.

[shivros]

🤖 Automated Review Panel

Reviewer A — GPT-5.6-terra

APPROVE. The generic action surface, legacy JSON fallback, deterministic rendering, and both GitHub/Iris dispatch paths align with the ticket.

Reviewer B — Gemini 3 Flash

APPROVE. No correctness or goal-alignment issue found.

Local verification: cargo build --all-targets; cargo test --all-targets (16 tests); cargo clippy --all-targets -- -D warnings; cargo fmt --all -- --check; git diff --check.

[shivros]

Auto-Merge Gate

Confidence: 0.93

The diff directly implements COD-466's approved generic-only http_post enrichment: defaulted headers, deterministic event/metadata template rendering, templated-body content-type behavior, and the unchanged legacy JSON-forwarding path. The required CI test check is successful; the runner's dual-model review panel found no unresolved correctness issues. Scope is limited to Rite's generic action configuration, dispatch paths, tests, and documentation.

No manual-review marker was found in COD-466's description or comments.