Add webhook action executor (HTTP POST)

[jaredzwick]

Summary

Ships the first concrete action executor for pypesdev/agents so agents can finally act, not just store. Per HIR-110 (Tier 1 #2 of HIR-105).

• Agent.actions storage on disk is unchanged (Vec<String>); each entry is now interpreted as a typed JSON Action spec at execution time. Strings that don't parse fall through as Unrecognized instead of breaking the loop.
• The first variant is webhook: HTTP POST a payload to a URL with optional headers; 2xx is success, anything else surfaces as a structured NonSuccessStatus error.
• New CLI: pypes agent <NAME> run runs every stored action through the executor pipeline.
• Tests use wiremock — no external services required to run cargo test.

What's in the diff

• src/executors/mod.rs — typed Action enum (#[serde(tag = "type")]), dispatch loop, parse helpers + tests.
• src/executors/webhook.rs — WebhookAction + execute_webhook + WebhookError. Header passthrough is the extent of auth handling (per the issue's "out of scope").
• src/main.rs — AgentCommands::Run wires the executor to the CLI.
• src/lib.rs — new, so examples/downstream crates can use pypes::executors.
• examples/webhook_executor.rs — end-to-end demo: spins up a tiny in-process axum receiver, runs the executor, prints the captured request.
• examples/webhook.json — minimal agent definition with one webhook action; pair with pypes agent webhook-demo run.
• README.md — new Action Executors → Webhook section + integration table flipped from experimental to shipped.
• CHANGELOG.md — Unreleased entry.
• Cargo.toml — reqwest (rustls, no native TLS) + wiremock dev-dep.

Example output

$ cargo run --example webhook_executor
→ stored action: {"headers":{"Authorization":"Bearer demo-token"},"payload":{"event":"agent.acted","n":1},"type":"webhook","url":"http://127.0.0.1:49207/hook"}
← webhook[0] status=200 body={"ok":true}
✓ mock receiver got 1 request(s):
    {"event":"agent.acted","n":1}

Test plan

• cargo build — clean.
• cargo test — 8 passed (4 new webhook executor + 2 new action parser + 2 pre-existing).
• cargo run --example webhook_executor — receives the POST and prints captured payload.
• CI pr_check workflow green.

Out of scope (separate tickets)

• Cron executor (Tier 1 add primitive ui #3).
• LLM executor (Tier 2).
• Auth/secrets resolution beyond raw header passthrough.

🤖 Generated with Claude Code