Replace custom validators with Click primitives

[seroperson]

Closes #1154

Summary

Replaces a layer of parameter-validation helpers across the CLI with Click's built-in primitives.

• validate_issue_id / require_valid_issue_id replaced with ContractIssueType Click param type.
• validate_ss58_address / require_valid_ss58 replaced with SS58AddressType Click param type.
• GitHub-issue-number parameters now has GitHubIssueType Click param type.
• RepoNameType - for --repo parameter, checks "owner/repo" string against REPO_PATTERN.
• --network gets type=NETWORK_CHOICE (the existing click.Choice(['finney','test','local']), so invalid networks fail at parse instead of at subtensor connect time.
• --pat gets envvar='GITTENSOR_MINER_PAT', the body-level os.environ.get(...) fallback is deleted.

Behavior change worth flagging: gitt i list --json --id 0 and gitt i submissions --json --id 0 no longer emit a JSON envelope on bad id, but it already covered by #1152 (which makes --json handle Click's exceptions).

How it looks like

User-visible error-message changes (sample, captured via Click test runner):

$ gitt i list --id 0
Usage: cli i list [OPTIONS]
Try 'cli i list --help' for help.

Error: Invalid value for '--id': 0 is not in the range 1<=x<=999999.

$ gitt admin set-owner not-an-ss58
Usage: cli admin set-owner [OPTIONS] NEW_OWNER
Try 'cli admin set-owner --help' for help.

Error: Invalid value for new_owner: Invalid SS58 address for new_owner: not-an-ss58

$ gitt miner post --network mainnet
Usage: cli miner post [OPTIONS]
Try 'cli miner post --help' for help.

Error: Invalid value for '--network': 'mainnet' is not one of 'finney', 'test', 'local'.

--help excerpts (auto-generated by Click from IntRange / Choice):

--id INTEGER RANGE     View a specific issue by ID  [1<=x<=999999]
--network [finney|test|local]
--pat TEXT             ... [env: GITTENSOR_MINER_PAT]

Testing

Dropped several basic tests which check issue_id parameter + dropped test which checks that incorrect issue_id parameter makes CLI response with a specific error format when used with --json (covered in #1152)