docs(agent): update agent install command with --target option

- Change `testsprite agent install` commands to use `--target` flag for specifying agents
- Update DOCUMENTATION.md examples to reflect new command syntax
- Modify README.md to replace old command usage with the new format
- Clarify onboarding instructions for different agents with consistent command style

Author: powxenv

DOCUMENTATION.md

@@ -86,7 +86,7 @@ This is the loop a coding agent runs on its own once you've onboarded it with `t
 
 ```bash
 # (one-time, per project) teach your agent the CLI
-testsprite agent install claude
+testsprite agent install --target claude
 
 # 1 — describe the behavior you want to guarantee, run it, wait
 testsprite test create --project proj_8f0f6 --type frontend \
@@ -109,10 +109,10 @@ Every artifact in the bundle shares one `snapshotId`; the CLI will not mix a fai
 `testsprite agent install` writes the TestSprite skills into your project so your coding agent knows the commands, the exit codes, and the failure-bundle layout — no prompt engineering required. It's a pure-local command: no network, no credentials.
 
 ```bash
-testsprite agent install claude-code
-testsprite agent install codex
-testsprite agent install cursor
-testsprite agent install kiro-cli
+testsprite agent install --target claude-code
+testsprite agent install --target codex
+testsprite agent install --target cursor
+testsprite agent install --target kiro-cli
 testsprite agent list
 testsprite agent status
 ```

README.md

@@ -131,7 +131,7 @@ Prefer to configure each step by hand (or learn the surface offline with `--dry-
 - 🤖 **Agent-shaped output.** `test failure get` returns **one bundle** — the failing step, its neighbors, screenshots, DOM snapshots, the test source, a root-cause hypothesis, and a recommended fix target — all sharing a single `snapshotId`. The CLI _refuses_ to stitch data from two different runs, so an agent never reasons over a frankenstein context.
 - ♻️ **A loop, not a one-shot.** `create → run → failure get → fix → rerun` — every pass is banked, not thrown away.
 - 📐 **Scriptable & deterministic.** Stable `--output json` contract, predictable [exit codes](./DOCUMENTATION.md#exit-codes), and a `--dry-run` that exercises the full code path offline with canned data.
-- 🔌 **One command to onboard your agent.** `testsprite agent install claude` drops a ready-made skill file into your repo so your coding agent knows how to drive the loop on its own.
+- 🔌 **One command to onboard your agent.** `testsprite agent install --target claude` drops a ready-made skill file into your repo so your coding agent knows how to drive the loop on its own.
 
 ## How it works