feat(skill): add read_skill tool for inline skill loading in plan mode

[yhemeo613]

Problem

run_skill is classified as a writer tool (ReadOnly() = false) because subagent skills can spawn isolated loops that call writer tools. This blocks ALL skill usage in plan mode, even for inline skills which are purely informational and have zero side effects.

Fixes #3491

Solution

Add read_skill — a read-only counterpart that only loads inline skill bodies:

• ReadOnly() returns true → callable in plan mode
• Rejects subagent skills with a clear message directing to run_skill instead
• Same name/arguments schema as run_skill for easy migration
• Added to SubagentMetaTools() exclusion list so sub-agents don't inherit it

Changes (7 files, +122/-5)

File	Change
internal/skill/tools.go	Add readSkillTool + NewReadSkillTool (+80 lines)
internal/skill/tools_test.go	4 new tests: inline load, subagent rejection, unknown name, empty args
internal/boot/boot.go	Register read_skill in tool registry
internal/agent/task.go	Add read_skill to meta-tool exclusion list
internal/skill/index.go	Mention read_skill in skills index header
internal/skill/skill_test.go	Update meta-tool list in test
internal/agent/task_test.go	Update meta-tool assertions

Testing

go test ./internal/skill/... ./internal/agent/... ./internal/boot/... ./internal/tool/...

All pass (97s agent suite, 4 new read_skill tests).