feat(executor): RunnerDefaults::from_env for CONVERGIO_RUNNER_DEFAULT

[Roberdan]

Problem

The daemon hard-coded `claude:sonnet` as the executor's default
runner kind. Operators using Copilot (or any other vendor) had no
way to flip the dispatcher's default short of editing
`crates/convergio-executor/src/defaults.rs` and rebuilding.

The runner crate already supports both vendors. The legacy
`/bin/echo` fallback was active for every dispatch because
`task.runner_kind` is unset on existing tasks and there was no
env-var override for the daemon-wide default.

Why

Without an env-driven knob there is no path from "I run Copilot
locally" to "the dispatcher routes my tasks through it" that
doesn't require recompiling. CONSTITUTION P1 (zero scaffolding):
the runner registry lives in code; expose it.

What changed

• New `RunnerDefaults::from_env()` reads:
  • `CONVERGIO_RUNNER_DEFAULT` — wire format `:`
    (e.g. `copilot:gpt-5.2`, `claude:opus`, `copilot:claude-opus`).
    Invalid values log a warning and the compiled-in default is used.
  • `CONVERGIO_RUNNER_PROFILE` — `standard` (default), `restricted`,
    `unrestricted`.
  • `CONVERGIO_DAEMON_URL` — base URL the spawned agent calls back
    to; defaults to `http://127.0.0.1:8420\`.
• Server wiring: the background loop in `main.rs` and the manual
  `POST /v1/dispatch` route both call
  `Executor::new(...).with_defaults(RunnerDefaults::from_env())`.
• Boot-time `tracing::info!` logs the resolved `runner_kind` /
  `runner_profile` so operators can confirm the daemon picked up
  their override.

The orthogonal switch `CONVERGIO_EXECUTOR_USE_RUNNER` (already in
`executor.rs`) is what flips the dispatcher off the legacy shell
template; the new env vars only take effect when a task either
sets its own `runner_kind` or `USE_RUNNER` is set.

Validation

• 4 new tests cover default / valid override / invalid fallback /
  daemon-url override, serialised through a `Mutex` because env
  vars are process-global.
• `cargo fmt --all -- --check` clean.
• `RUSTFLAGS="-Dwarnings" cargo clippy --workspace --all-targets -- -D warnings` clean.
• `RUSTFLAGS="-Dwarnings" cargo test --workspace` — 1047 passed,
  0 failed.
• Pre-commit hooks (file-size, fmt, clippy, context-budget) all
  green.

End-to-end smoke (out-of-band): launchd plist with
`CONVERGIO_RUNNER_DEFAULT=copilot:gpt-5.2` +
`CONVERGIO_EXECUTOR_USE_RUNNER=1` + matching `PATH` entry for
`copilot` / `gh` correctly produces 20+ `copilot -p ...` child
processes carrying the per-task prompt + Tier-3 graph context-pack
from `convergio-runner`. Processes stopped before any PR was
opened — auto-dispatch is an operator decision, not enabled here.

Impact

• `convergio-executor` + `convergio-server` only.
• New public method `RunnerDefaults::from_env()`. Existing
  `RunnerDefaults::default()` unchanged.
• Boot log adds two structured fields. No HTTP-shape changes.

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[Roberdan]

CI status update (2026-05-07)

All 3 checks passed on head commit 559b6e3:

Check	Result
fmt + clippy + test	✅ success
cargo deny + audit	✅ success
Docs drift (advisory)	✅ success

PR is 1 day old — no age flag. Ready for review when a maintainer is available.

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