feat: add ability to edit harness configuration after creation

### Description

There is no way to edit a harness's configuration after the initial `agentcore add harness` flow. If a user needs to change the system prompt, swap a model, add/remove tools, or fix a misconfigured gateway tool, they have no CLI-native path to do so.

This is especially painful for **gateway tools within a harness**. When a gateway is added during harness creation, the ARN and auth config are written to `app//harness.json` — a file users don't know exists. If the gateway ARN or OAuth config is wrong, deployments fail with no discoverable way to fix it from the CLI.

### Current Behavior

`HarnessPrimitive` only implements `add()` and `remove()`. All harness configuration is written once during `add()`:

- **Harness spec** → `app//harness.json` (via `configIO.writeHarnessSpec()`)
- **System prompt** → `app//system-prompt.md`
- **Project reference** → `agentcore/agentcore.json`

Properties that cannot be changed after creation:
- System prompt
- Model provider / model ID
- Tools (browser, code_interpreter, MCP servers, **gateway ARN + auth config**)
- Max iterations, max tokens, timeout
- Truncation strategy
- Network mode, subnets, security groups
- Container URI / Dockerfile path
- Authentication configuration

The only workarounds are:
1. Manually edit `app//harness.json` (undiscoverable)
2. `agentcore remove harness` + `agentcore add harness` (destructive — loses customizations)

### Expected Behavior

Users should be able to edit harness configuration after creation. For example:

```
agentcore edit harness
agentcore edit harness --name
```

This could re-enter the wizard with current values as defaults, or open the config in an editor, or support targeted flag-based edits — whatever fits the CLI's UX patterns best.

### Relevant Code

- **Harness spec written (once):** `src/cli/primitives/HarnessPrimitive.ts` line ~177
- **System prompt written (once):** `src/cli/primitives/HarnessPrimitive.ts` lines ~178-183
- **Gateway tool config built during add:** `src/cli/primitives/HarnessPrimitive.ts` lines 110-138
- **Harness spec file path:** `app//harness.json` (via `PathResolver.getHarnessConfigPath()`)
- **Tools passed to API unchanged on deploy:** `src/cli/operations/deploy/imperative/deployers/harness-mapper.ts` lines 248-254
- **No `update()` or `edit()` method on `HarnessPrimitive` or `BasePrimitive`**

### Impact

Harness iteration is a core development workflow. Users frequently need to tweak system prompts, swap models, fix gateway ARNs, or add/remove tools. Currently, any mistake during initial creation forces users out of the CLI workflow — either manually editing files they don't know about, or destroying and recreating the entire harness.