feat: adds support for private key

[stalniy]

Why

Closes #4

What

1. Declarative validation of env vars with zod
2. Adds support to specify private key instead of wallet mnemonic

[Copilot]

Pull request overview

This PR introduces a unified “wallet secret” configuration that supports either a mnemonic or a raw secp256k1 private key, and updates the CLI env parsing to use Zod-based declarative validation.

Changes:

• Added walletSecret support (mnemonic or private key) with validation and wallet creation logic.
• Reworked CLI env parsing around a Zod schema and replaced MNEMONIC with WALLET_SECRET.
• Updated tests, README, and dependencies to reflect the new configuration model.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/validation.ts	Adds validateWalletSecret and imports secp256k1 key validation helper.
src/validation.test.ts	Replaces integer parsing tests with wallet secret validation tests.
src/hermes-client.ts	Switches from mnemonic-only wallet creation to mnemonic/privateKey signer creation; renames endpoint security flag.
src/hermes-client.test.ts	Updates config shape and adds private-key initialization test coverage; removes interval parsing tests.
src/cli-commands/create-command-builder.ts	Improves CLI config error formatting (multi-line output).
src/cli-commands/create-command-builder.test.ts	Updates expectations for new WALLET_SECRET parsing shape.
src/cli-commands/command-config.ts	Replaces manual env parsing with a Zod schema and parses WALLET_SECRET.
src/cli-commands/command-config.test.ts	Updates env parsing tests for WALLET_SECRET and new defaulting behavior.
package.json	Adds zod dependency.
package-lock.json	Locks zod and updates some dev dependency resolutions.
README.md	Documents WALLET_SECRET env var in place of MNEMONIC.

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[Copilot]

Pull request overview

Copilot reviewed 10 out of 11 changed files in this pull request and generated 4 comments.

Comments suppressed due to low confidence (1)

README.md:179

• The env var table is updated to WALLET_SECRET, but the README still contains other references/examples using MNEMONIC and wording like "Create wallet from mnemonic" later in the document. To avoid confusing users, update the remaining occurrences to the new WALLET_SECRET format (and document the privateKey option consistently).

| `WALLET_SECRET` | Yes | - | Either `privateKey:<private key in hex format>` or `mnemonic:<12/24 words>` |
| `HERMES_ENDPOINT` | No | `https://hermes.pyth.network` | Pyth Hermes API |
| `UPDATE_INTERVAL_MS` | No | `300000` | Update interval (5 min) |
| `GAS_PRICE` | No | `0.025uakt` | Gas price |
| `DENOM` | No | `uakt` | Token denomination |

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