feat(skills): configure upload size limit

[evgyur]

Summary

• Adds skills.max_upload_size_mb to the JSON/JSON5 config with the current 20 MiB behavior as the default.
• Adds GOCLAW_SKILLS_MAX_UPLOAD_SIZE_MB as an environment override.
• Clamps the effective global upload size to the documented safety range and wires it into /v1/skills/upload.

Test plan

• gofmt -w internal/config/defaults.go internal/config/config.go internal/config/config_load.go internal/config/skills_config_test.go internal/http/skills.go internal/http/skills_upload.go internal/http/skills_upload_test.go
• go test ./internal/config -run 'TestSkillsConfig|TestLoadSkillsMaxUploadSize' -count=1
• go test ./internal/http -run 'TestHandleUpload_UsesConfiguredMaxUploadSize|TestHandleUpload_AutoInstallsMissingDepsAndKeepsSkillActive' -count=1
• go test ./internal/config ./internal/http ./cmd -count=1
• git diff --check

Closes #1153

[mrgoonie]

Review result: closing as superseded.

The requested skill upload-size configuration already exists on dev as a broader implementation than this PR:

• Implemented by commit 1296031d / merge 14b32bd2 (feat(skills): configure upload size limits).
• First shipped in beta release v3.12.0-beta.1, whose release notes include Merge pull request #61 ... issue-28-skill-upload-size and feat(skills): configure upload size limits.
• First stable release containing it: v3.13.0.

Current dev covers the requested behavior and more: skills.max_upload_size_mb, GOCLAW_SKILLS_MAX_UPLOAD_SIZE_MB, tenant system_configs["skills.max_upload_size_mb"], SKILL.md frontmatter max_upload_size_mb, 1-500 MB clamping, HTTP upload enforcement, Web UI setting, docs, and tests.

No fixes pushed. CI was not run because this PR is not being merged; it is currently conflicted (mergeStateStatus: DIRTY) and duplicates shipped functionality.

Unresolved questions: none.