feat(config): add YAML safety mode defaults

[Sayt-0]

Summary

• add declarative safety defaults to team runtime, individual agents, user settings, and aliases
• enforce CLI, user-owned, author-owned, and persisted-session precedence without weakening user preferences
• preserve legacy YOLO/yolo behavior and validate canonical YAML values
• update the schema, documentation, example configuration, sandbox forwarding, and session/API coverage

Closes #3837

Precedence

Priority	Source
1	explicit --safety
2	explicit --yolo
3	alias safety or legacy yolo
4	settings.safety or legacy settings.YOLO
5	selected agents.<name>.safety
6	runtime.safety
7	historical unset behavior

At the same scope, safety wins over the legacy boolean alias. Resumed sessions keep their persisted mode unless an explicit CLI override is supplied.

Acceptance criteria

Expectation	Implementation
Team-wide default	runtime.safety is propagated through the loaded team
Per-agent override	the initially selected agent's safety wins over the runtime default
User-wide protection	settings and alias defaults outrank local, URL, and OCI author configuration
CLI precedence	explicit flag state is tracked independently from defaults
Resume behavior	only explicit CLI safety flags replace persisted state
Agent/session inheritance	switches, handoffs, spawned sessions, and delegated sessions preserve or inherit the effective mode
Compatibility	settings.YOLO, alias yolo, and --yolo remain autonomous aliases
Validation	only strict, balanced, and autonomous are accepted in new YAML fields
Schema and docs	schema, CLI/config docs, headless guidance, and safety example are updated
Tests	precedence, validation, selection, resume, API, sandbox, and persistence paths are covered

Validation

• env -u HTTP_PROXY -u HTTPS_PROXY -u http_proxy -u https_proxy go test ./...
• go test -race -count=1 ./pkg/server -run 'TestAuthorSafetyDefault|TestSessionManager_SetSessionAgentModel'
• golangci-lint run
• go run ./lint .
• go build ./...
• go mod tidy -diff
• jq empty agent-schema.json
• npx --yes markdownlint-cli2@0.22.1
• ./scripts/docs-check-canonical.sh
• ./scripts/docs-check-llms-txt.sh

Known limitation

An API session created without a safety choice receives author defaults when its first run starts. If the server restarts before that first run, it retains the conservative historical unset behavior rather than being re-defaulted later.