feat: clippy/lint output grouping by rule

[bug-ops]

Parent

Epic: #426

Summary

Group clippy warnings/errors by lint rule, showing count and first occurrence instead of repeating the full diagnostic for each instance.

Expected Savings

70-90% on large codebases with many warnings.

Behavior

# Before: 25 full clippy diagnostics, each 5-10 lines

# After:
clippy::needless_return (12 occurrences)
  → crates/zeph-core/src/agent/mod.rs:45
clippy::unused_imports (8 occurrences)
  → crates/zeph-llm/src/claude.rs:3
error[E0599]: method not found (1)
  → crates/zeph-tools/src/shell.rs:112:
    | self.foo()
    | ^^^ method not found in `ShellExecutor`

warnings: 20, errors: 1

Rules

• Errors: always show full diagnostic (agent needs details to fix)
• Warnings: group by rule, show count + first location
• On zero warnings/errors: just the summary line

Implementation

• ClippyOutputFilter matching cargo clippy
• Parse warning[rule] and error[E####] headers
• Group warnings, preserve full error blocks
• Emit summary counts

Acceptance Criteria

• Warnings grouped by rule with count
• Errors shown in full
• Clean output → summary only
• Unit tests