fix(config): add serde(default) to TeeConfig fields

[fkztw]

Summary

• TeeConfig fields lack #[serde(default)], causing the entire Config deserialization to fail when users specify a [tee] section in config.toml but omit some fields (e.g. max_file_size)
• Since call sites use Config::load().map(...).unwrap_or_default() to silently handle errors, all user settings (including hooks.exclude_commands) are silently ignored with no warning
• Add #[serde(default = "...")] to every TeeConfig field so partial [tee] sections deserialize correctly
• Add tests for partial and empty deserialization

Reproduction

# ~/.config/rtk/config.toml (Linux) or ~/Library/Application Support/rtk/config.toml (macOS)
[hooks]
exclude_commands = ["curl", "helm"]

[tee]
enabled = true
mode = "failures"
max_files = 20
# max_file_size is omitted — this causes the ENTIRE config to fail silently

$ rtk config
Config: ~/Library/Application Support/rtk/config.toml

Error: TOML parse error at line 35, column 1
   |
35 | [tee]
   | ^^^^^
missing field `max_file_size`

When rtk config is not called directly, Config::load() fails silently and falls back to defaults — all user settings are ignored without any warning.

Test plan

• cargo fmt --all --check — passed
• cargo clippy --all-targets — no new warnings
• cargo test --all — 1116 passed, 3 ignored
• New test_tee_config_partial_deserialize — verifies omitting max_file_size does not fail
• New test_tee_config_empty_deserialize — verifies a completely empty [tee] section uses defaults

🤖 Generated with Claude Code

[CLAassistant]

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[aeppling]

Hey

We are cleaning up the codebase and improving the project structure for better onboarding. As part of this effort, PR #826 reorganizes src/ from a flat layout into subfolders.

No logic changes — only file moves and import path updates.

What you need to do

Rebase your branch on develop when receiving this comment:

git fetch origin && git rebase origin/develop

Git detects renames automatically. If you get import conflicts, update the paths:

use crate::git;        // now: use crate::cmds::git::git;
use crate::tracking;   // now: use crate::core::tracking;
use crate::config;     // now: use crate::core::config;
use crate::init;       // now: use crate::hooks::init;
use crate::gain;       // now: use crate::analytics::gain;

Need help rebasing? Tag @aeppling