refactor(#2806): runtime cleanup — Ansi consolidation, suspending ses…

[Skobeltsyn]

…sion send, BuildInfo version source

Three independent code smells in the runtime + MCP layers, fixed in one behavior-preserving pass.

Ansi consolidation (LiveShow.kt)

• New internal agents_engine.runtime.Ansi object owns ESC / RESET / ERASE_LINE constants.
• AnsiColor.code values reference Ansi.ESC instead of embedding the raw escape byte; AnsiColor.wrap terminates with Ansi.RESET instead of an inline [0m.
• Spinner clear in runWithSpinner uses Ansi.ERASE_LINE instead of raw [2K.
• Deleted the dead AnsiColor.Companion.RESET const — never referenced, while wrap hardcoded its own copy.
• Pinned by new AnsiTest.

Session emitter backpressure (#2806 acceptance criterion 2)

• Bracket events (Completed / Failed) on Agent / Pipeline / Branch / Parallel session extensions switched from non-suspending trySend to suspending send. They run inside scope.launch { }; suspending is safe and guarantees terminal events are never dropped.
• The inner per-token emitter stays on trySend (the typealias is a non-suspending lambda), but now logs a JUL warning with the event class name + agent id on failure — silent drops become visible.

MCP version single-source-of-truth

• New internal agents_engine.internal.BuildInfo reads Implementation-Version from the JAR manifest, falling back to "dev" on non-sealed classpaths.
• tasks.jar { manifest { attributes(...) } } in build.gradle.kts now stamps the manifest from project.version.
• McpServer.SERVER_VERSION, McpClient.CLIENT_VERSION, McpRunner.VERSION all forward to BuildInfo.version. The three had drifted independently ("0.1.3" / "0.1.3" / "0.3.0").
• McpServerInfoSnapshotTest updated to assert against BuildInfo.version instead of the stale "0.1.3" literal.

Optional acceptance items (per-flag handler extraction in resolveConfig, URI-based host parsing in McpServer.hostOnly) deferred — not blocking the bundle and out of scope for the targeted maintainability fix.