docs(openspec): add FN-007 Git Connection Model change

- Add proposal, design, specs, and tasks for git connection model
- Include provider adapter, credential storage, SSH key lifecycle specs
- Add repository connection API and git operations specifications

openspec/changes/git-connection-model/specs/ssh-key-lifecycle/spec.md

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+## ADDED Requirements
+
+### Requirement: SSH keys are generated per repository
+The system SHALL generate Ed25519 SSH key pairs for each repository connection.
+
+#### Scenario: Generate key pair
+- **WHEN** a repository connection is created
+- **THEN** the system generates an Ed25519 key pair
+- **AND** stores the private key encrypted
+- **AND** returns the public key for deploy key registration
+
+#### Scenario: Retrieve public key
+- **WHEN** a user requests the public key for a connection
+- **THEN** the system returns the SSH public key string
+
+### Requirement: SSH keys support lifecycle operations
+The system SHALL support rotating and revoking SSH keys.
+
+#### Scenario: Rotate key
+- **WHEN** a user rotates an SSH key
+- **THEN** the system generates a new key pair
+- **AND** updates the deploy key on the provider
+- **AND** deletes the old key pair
+
+#### Scenario: Revoke key
+- **WHEN** a connection is deleted
+- **THEN** the system deletes the deploy key from the provider
+- **AND** securely deletes the local key pair