Feature request: configurable default flags for `rtk read` and `rtk grep`

Problem

When using RTK with an AI assistant (Claude Code), the flags -l aggressive
(for rtk read) and --ultra-compact (for rtk grep) dramatically improve
token savings — but they must be specified manually on every call.

Measured impact over real sessions:

• rtk read without flag: ~1-6% savings (101 calls wasted)
• rtk read -l aggressive: ~80% savings
• rtk grep without flag: ~50% savings
• rtk grep --ultra-compact: ~80% savings

Because these flags cannot be set as defaults, savings depend entirely on
the AI assistant's behavior — which is inconsistent across sessions due to
context resets. Average adoption sits at ~55% despite explicit instructions
in CLAUDE.md.

Proposed Solution

Add a [defaults] section to config.toml allowing users to set default
flags per command:

[defaults]
read_level = "aggressive"       # equivalent to -l aggressive on every rtk read
grep_ultra_compact = true       # equivalent to -u on every rtk grep

Start by tracing how rtk read and rtk grep currently parse their flags and how configuration is loaded from config.toml. Implement the proposed [defaults] settings for read_level and grep_ultra_compact, then verify that the configured defaults affect both commands as described and that the documented behavior is covered by tests.