feat(tools): tool registry, file executor, doom-loop detection

[bug-ops]

Summary

• Add ToolRegistry with typed ToolDef definitions for 7 built-in tools (bash, read, edit, write, glob, grep, web_scrape)
• Add sandboxed FileExecutor for file operations with path traversal protection
• Extend ToolExecutor trait with ToolCall struct and execute_tool_call() for structured tool invocation
• Update CompositeExecutor to aggregate definitions and route tool calls by tool_id
• Include tool catalog in system prompt via format_for_prompt()
• Raise iteration cap from hardcoded 3 to configurable 10 (max_tool_iterations in TOML / ZEPH_AGENT_MAX_TOOL_ITERATIONS env)
• Add doom-loop detection: break on 3 consecutive identical tool outputs
• Add context budget check at 80% threshold

Closes #239,Closes #240,Closes #241,Closes #242,Closes #243,Closes #244,Closes #245

Test plan

• 1304 unit tests pass (10 new tests for registry, file executor, routing, doom-loop, budget check)
• Security audit: sandbox bypass fixes verified (write, glob, grep paths)
• Performance review: no high-impact issues
• clippy zero warnings, fmt clean

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.83558% with 68 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/zeph-tools/src/file.rs	92.79%	26 Missing ⚠️
crates/zeph-core/src/agent.rs	68.11%	22 Missing ⚠️
crates/zeph-tools/src/composite.rs	83.82%	11 Missing ⚠️
crates/zeph-core/src/context.rs	75.00%	4 Missing ⚠️
crates/zeph-tools/src/executor.rs	85.71%	3 Missing ⚠️
crates/zeph-tools/src/registry.rs	99.50%	1 Missing ⚠️
src/main.rs	0.00%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main     #246      +/-   ##
==========================================
+ Coverage   80.86%   81.15%   +0.29%     
==========================================
  Files          79       81       +2     
  Lines       23239    23970     +731     
==========================================
+ Hits        18792    19454     +662     
- Misses       4447     4516      +69     

Files with missing lines	Coverage Δ
crates/zeph-core/src/config.rs	98.92% <100.00%> (+<0.01%)	⬆️
crates/zeph-tools/src/registry.rs	99.50% <99.50%> (ø)
src/main.rs	69.96% <0.00%> (-0.06%)	⬇️
crates/zeph-tools/src/executor.rs	97.34% <85.71%> (-2.66%)	⬇️
crates/zeph-core/src/context.rs	97.65% <75.00%> (-1.86%)	⬇️
crates/zeph-tools/src/composite.rs	92.41% <83.82%> (-7.59%)	⬇️
crates/zeph-core/src/agent.rs	69.05% <68.11%> (-0.10%)	⬇️
crates/zeph-tools/src/file.rs	92.79% <92.79%> (ø)

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