feat(mecatui): own configurable terminal titles

[jbeda]

Implementation

Implements the approved terminal-title contract from Plan / Interface PR #1616.

Delivered

• Mecatui owns title rendering and emits deduplicated OSC 0 updates instead of Bubble Tea OSC 2 output.
• Adds strict user-global terminal_title.enabled and terminal_title.template settings over safe plain-text status facts, plus display-width elide.
• Preserves explicit --terminal-title / MECATUI_NO_TERMINAL_TITLE precedence and conditional cleanup.
• Removes session handles from default header/title presentation, preserves opt-in template access, and enables read-only /session details during active runs.
• Excludes private workspace paths and StatusML helpers from title templates; includes race, configuration, terminal-control, and live-run regressions.

Plan / Interface PR: #1616
Approved baseline: 2a0c9cb11bedc6ef88503f504bbd4f94a6d31690
Interfaces match approved contract: Yes

Validation

• task lint
• task test
• task docs
• task site:build
• go run ./cmd/mecademo
• AC coverage: 14/14 for mecatui-terminal-title-controller

task ac-trace-strict remains red only on pre-existing, unrelated accepted-plan proofs in operator-defined-llm-providers and sdk-typescript-local; explicitly waived by the operator for this PR.

Tracking: #1460 and #1606.