feat(extensions): add extension loader infrastructure (#3)

## Summary

- Adds a modular extension system for Lightning.Pub enabling third-party plugins
- Provides isolated SQLite databases per extension for data safety
- Implements ExtensionContext API for accessing Lightning.Pub services (payments, Nostr, storage)
- Supports RPC method registration with automatic namespacing
- Includes HTTP route handling for protocols like LNURL
- Event routing for payment receipts and Nostr events
- Comprehensive documentation with architecture overview and working examples

## Key Components

- `src/extensions/types.ts` - Core extension interfaces
- `src/extensions/loader.ts` - Extension discovery, loading, and lifecycle management
- `src/extensions/context.ts` - Bridge between extensions and Lightning.Pub services
- `src/extensions/database.ts` - SQLite isolation with WAL mode
- `src/extensions/README.md` - Full documentation with examples

## ExtensionContext API

| Method | Description |
|--------|-------------|
| `getApplication()` | Get application info |
| `createInvoice()` | Create Lightning invoice |
| `payInvoice()` | Pay Lightning invoice |
| `getLnurlPayInfo()` | Get LNURL-pay info for a user (enables Lightning Address/zaps) |
| `sendEncryptedDM()` | Send Nostr DM (NIP-44) |
| `publishNostrEvent()` | Publish Nostr event |
| `registerMethod()` | Register RPC method |
| `onPaymentReceived()` | Subscribe to payment callbacks |
| `onNostrEvent()` | Subscribe to Nostr events |

## Test plan

- [x] Review extension loader code for correctness
- [x] Verify TypeScript compilation succeeds
- [x] Test extension discovery from `src/extensions/` directory
- [x] Test RPC method registration and routing
- [x] Test database isolation between extensions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: boufni95 <boufni95@gmail.com>
Co-authored-by: Patrick Mulligan <patjmulligan@protonmail.com>
Reviewed-on: #3

proto/others/walletkit.proto

@@ -1172,6 +1172,12 @@ message PendingSweep {
     The deadline height used for this output when perform fee bumping.
     */
     uint32 deadline_height = 14;
+
+    /*
+    The block height which the input's locktime will expire at. Zero if the
+    input has no locktime.
+    */
+    uint32 maturity_height = 15;
 }
 
 message PendingSweepsRequest {
@@ -1188,9 +1194,8 @@ message BumpFeeRequest {
     // The input we're attempting to bump the fee of.
     lnrpc.OutPoint outpoint = 1;
 
-    // Optional. The deadline in number of blocks that the input should be spent
-    // within. When not set, for new inputs, the default value (1008) is used;
-    // for existing inputs, their current values will be retained.
+    // Optional. The conf target the underlying fee estimator will use to
+    // estimate the starting fee rate for the fee function.
     uint32 target_conf = 2;
 
     /*
@@ -1217,7 +1222,7 @@ message BumpFeeRequest {
 
     /*
     Optional. Whether this input will be swept immediately. When set to true,
-    the sweeper will sweep this input without waiting for the next batch.
+    the sweeper will sweep this input without waiting for the next block.
     */
     bool immediate = 6;
 
@@ -1230,6 +1235,12 @@ message BumpFeeRequest {
     retained.
     */
     uint64 budget = 7;
+
+    // Optional. The deadline delta in number of blocks that the output
+    // should be spent within. This translates internally to the width of the
+    // fee function that the sweeper will use to bump the fee rate. When the
+    // deadline is reached, ALL the budget will be spent as fees.
+    uint32 deadline_delta = 8;
 }
 
 message BumpFeeResponse {
@@ -1243,7 +1254,8 @@ message BumpForceCloseFeeRequest {
     lnrpc.ChannelPoint chan_point = 1;
 
     // Optional. The deadline delta in number of blocks that the anchor output
-    // should be spent within to bump the closing transaction.
+    // should be spent within to bump the closing transaction. When the
+    // deadline is reached, ALL the budget will be spent as fees
     uint32 deadline_delta = 2;
 
     /*
@@ -1270,6 +1282,10 @@ message BumpForceCloseFeeRequest {
     transaction of the force closed channel otherwise the fee bumping will fail.
     */
     uint64 budget = 5;
+
+    // Optional. The conf target the underlying fee estimator will use to
+    // estimate the starting fee rate for the fee function.
+    uint32 target_conf = 6;
 }
 
 message BumpForceCloseFeeResponse {
@@ -1426,6 +1442,16 @@ message FundPsbtRequest {
 
     // The max fee to total output amount ratio that this psbt should adhere to.
     double max_fee_ratio = 12;
+
+    // The custom lock ID to use for the inputs in the funded PSBT. The value
+    // if set must be exactly 32 bytes long. If empty, the default lock ID will
+    // be used.
+    bytes custom_lock_id = 13;
+
+    // If set, then the inputs in the funded PSBT will be locked for the
+    // specified duration. The lock duration is specified in seconds. If not
+    // set, the default lock duration will be used.
+    uint64 lock_expiration_seconds = 14;
 }
 message FundPsbtResponse {
     /*