fix(grep): grep context blowup from trajectory files

[Dong1017]

Summary

Fix a grep failure mode where broad searches could pull large .cache/*.trajectory.jsonl contents into tool output and blow up the agent context window.

This change keeps the fix minimal and localized to tools/fs/grep.go.

Root Cause

grep recursively searched the workspace without guarding against runtime trajectory files under .cache/.
Those files can contain large conversation traces, and grep previously returned unbounded matching output, which could flood model context.

Changes

• Skip .cache/*.trajectory.jsonl during directory walk
• Keep explicit file-path search working for trajectory files
  • if the user directly targets a specific trajectory file, grep still allows it
• Add head_limit to cap returned match count
  • default: 20
  • max: 100
• Truncate overly long matched lines before returning them
• Add focused tests for:
  • skipping trajectory files during recursive search
  • allowing explicit trajectory file search
  • head_limit truncation
  • long-line truncation
  • no-match behavior

Scope

Intentionally not changed in this PR:

• no global tool-result truncation
• no context-manager changes
• no UI truncation changes
• no trace writer changes
• no broader .cache exclusion beyond *.trajectory.jsonl

Why this approach

This is the smallest fix that addresses the current incident while preserving existing grep behavior as much as possible.

It blocks the known self-referential trace-file case and adds minimal output guards so grep cannot return effectively unbounded content.

Validation

Ran:

• gofmt -w tools/fs/grep.go tools/fs/grep_test.go
• go test ./tools/fs
• git diff --check

Test result:

• ok github.com/vigo999/ms-cli/tools/fs

Risk

Low.

The behavior change is limited to:

• recursive search skipping .cache/*.trajectory.jsonl
• bounded grep output
• truncation of unusually long matched lines