Default chunk task run to create a new branch

[hidetaka-cci]

• Default --new-branch to true so chunk task run sends create-new-branch: true when the flag is omitted, matching the Web UI.
• Users who want to commit on the checkout branch must pass --new-branch=false explicitly.
• Update docs/CLI.md and tests for the new default.

Closes #432

Motivation

chunk task run without --new-branch could commit agent changes directly to default_branch (often main) with no confirmation. The Web UI always sets create-new-branch: true; chunk-cli defaulted to false, which is unsafe for users following task config → task run.

Behavior change

Invocation	Before	After
chunk task run ... (no branch flag)	create-new-branch: false	create-new-branch: true
chunk task run ... --new-branch	create-new-branch: true	create-new-branch: true (unchanged)
chunk task run ... --new-branch=false	N/A (was default false)	create-new-branch: false

Breaking change: Scripts that depended on the old silent default must add --new-branch=false.

Example

# Default: agent creates a new branch (same as Web UI)
chunk task run --definition dev --prompt "Fix the flaky test"

# Opt out: commit on the checkout branch (previous default)
chunk task run --definition dev --prompt "Fix the flaky test" --new-branch=false

Changes

Area	Change
internal/cmd/task.go	--new-branch default false → true; updated flag help
docs/CLI.md	Flag tree + Behavior Decisions note for default and opt-out
acceptance/task_test.go	TestTaskRunHappyPath expects true; TestTaskRunNewBranchFalse for --new-branch=false
internal/task/task_test.go	Happy path uses NewBranch: true; add TestTriggerRunNoNewBranch

Out of scope

• TTY confirmation prompt or --yes / --force for direct commits
• Server-side auto-override of create-new-branch
• Changes to --branch or default_branch resolution

Test plan

• go test -race ./...
• TestTaskRunHappyPath — default payload sends create-new-branch: true
• TestTaskRunNewBranchFalse — --new-branch=false sends create-new-branch: false
• TestTriggerRunNoNewBranch — unit coverage for explicit NewBranch: false