feat(cli): add config init and config show subcommands

[geoffjay]

Adds agent config init and agent config show subcommands to the CLI, backed by agentd_common::config::load().

• config init writes a fully-commented config.toml template to the XDG config path; refuses to overwrite without --force
• config show prints the merged config (defaults + file + env vars) as TOML or --json; --raw prints the on-disk file only
• 4 unit tests: template TOML validity, struct deserialisation, overwrite guard logic

Closes #1195

[geoffjay]

Review: feat(cli): add config init and config show subcommands

Stack position: This PR (issue-1195) is stacked on PR #1202 (issue-1199) which is stacked on PR #1201 (issue-1194, needs-rework). It cannot merge until #1201 is fixed and the chain restacks. After #1201's rework lands, the template and any tests that reference field names (e.g. AGENTD_HISTORY_SIZE) will need updating.

The implementation itself is solid and well-structured. One test-quality issue needs a fix before this merges.

---

Blocking

Overwrite-guard tests don't call cmd_init — they test a boolean expression

#[test]
fn init_refuses_overwrite_without_force() {
    let dir = tempfile::tempdir().unwrap();
    let path = dir.path().join("config.toml");
    std::fs::write(&path, "# existing").unwrap();

    // Simulate the guard logic (path.exists() && !force)
    assert!(path.exists());
    let would_error = path.exists() && !false;  // always true
    assert!(would_error, "should refuse overwrite without --force");
}

cmd_init is never called. The test writes a file, then asserts that true && true == true. It would pass whether cmd_init bails, silently overwrites, or panics. The same holds for init_allows_overwrite_with_force — path.exists() && !true is false by definition.

The fix is to test the actual function. Since cmd_init is private, expose it as pub(crate) and use AGENTD_CONFIG to redirect it at the tempdir:

#[test]
fn init_refuses_overwrite_without_force() {
    let dir = tempfile::tempdir().unwrap();
    let path = dir.path().join("config.toml");
    std::fs::write(&path, "# existing").unwrap();
    std::env::set_var("AGENTD_CONFIG", path.to_str().unwrap());

    let result = cmd_init(false);
    std::env::remove_var("AGENTD_CONFIG");

    assert!(result.is_err());
    let msg = result.unwrap_err().to_string();
    assert!(msg.contains("--force"), "error should mention --force, got: {msg}");
}

#[test]
fn init_allows_overwrite_with_force() {
    let dir = tempfile::tempdir().unwrap();
    let path = dir.path().join("config.toml");
    std::fs::write(&path, "# existing").unwrap();
    std::env::set_var("AGENTD_CONFIG", path.to_str().unwrap());

    let result = cmd_init(true);
    std::env::remove_var("AGENTD_CONFIG");

    assert!(result.is_ok());
    let written = std::fs::read_to_string(&path).unwrap();
    assert!(written.contains("[general]"), "template should have been written");
}

(If env-var mutation in tests needs serialisation, add an ENV_LOCK-style mutex as in agentd-common's config tests.)

---

Non-blocking observations

1. config_file_path() called twice in cmd_show's None arm

None => {
    eprintln!("{}", "No config file found.".yellow());
    eprintln!(
        "Run `agent config init` to create one at {}",
        config_file_path()          // ← second XDG lookup
            .map(|p| p.display().to_string())
            .unwrap_or_else(|| "~/.config/agentd/config.toml".to_string())
    );
}

path (the first lookup) is already in scope. Both calls resolve the same value. Replace the second call with path.as_ref().map(|p| p.display().to_string()).unwrap_or_else(...).

2. config_file_path() local wrapper is unnecessary

fn config_file_path() -> Option<PathBuf> {
    agentd_common::config::config_file_path()
}

Two call sites in the file could simply call agentd_common::config::config_file_path() directly, or add use agentd_common::config::config_file_path; at the top. The wrapper adds a layer of indirection for no gain.

3. Template will need updating after #1201's rework

The template documents AGENTD_HISTORY_SIZE for hook.history_size. If #1201's rework resolves the AGENTD_HISTORY_SIZE collision (hook vs monitor) by renaming to AGENTD_HOOK_HISTORY_SIZE, this comment and any new monitor fields need to be reflected here. Note it in the PR for whoever does the restack.

4. TOCTOU in cmd_init — the path.exists() check and fs::write are not atomic. Acceptable for a config-init command; no action needed.

---

What's correct

• template_is_valid_toml and template_deserialises_to_agentd_config are meaningful regressions tests — good
• All template field values match the AgentdConfig compiled defaults ✓
• execute() is sync (correct — no async I/O; consistent with auth command pattern) ✓
• anyhow::Context used on both Option (via the ? in cmd_init) and Result (serialization) — correct ✓
• Error messages are user-friendly and actionable ✓
• colored::Colorize for the success line — consistent with existing CLI output ✓
• toml = { workspace = true } and agentd-common = { path = "../common" } — dependency additions are correct ✓
• Config dispatch in main.rs is synchronous (command.execute(cli.json)? without .await) — correct ✓

---

Summary: Fix the overwrite-guard tests to call cmd_init rather than evaluate boolean literals. Everything else is clean. Hold for parent #1201 restack.

[geoffjay]

This change is part of the following stack:

• feat(common): add config module with TOML schema and layered loading #1201
  • test(common): add integration tests for layered config precedence #1202
    • feat(cli): add config init and config show subcommands #1203 ◀
      • refactor: migrate services with existing config.rs to shared TOML config #1204
        • feat: add config structs for services with inline env var config #1205
          • feat(config): add ValidateConfig trait and per-service validation #1224
            • docs: add configuration system documentation #1207

_{Change managed by git-spice.}