[Feature] Add Confluence Cloud provider with CQL search

[mbe24]

Problem

99problems currently supports GitHub, GitLab, Jira, and Bitbucket (Cloud and Datacenter), but not Confluence.

Teams that store architecture/decision context in Confluence cannot retrieve that context through 99problems, which limits cross-system task preparation for humans and agents.

Design

Implement Confluence Cloud provider (selfhosted will follow in future issue) using the existing conversation schema:

1. map each Confluence page as an issue-like conversation,
2. support raw CQL search via -q,
3. support page fetch by Confluence page ID.

Confluence v1 boundaries:

1. Cloud only,
2. no PR mode (--type pr rejected),
3. --repo reused as Confluence space key,
4. --url required for tenant base URL.

Scope

• Add source adapter under src/source/confluence/.
• Wire confluence platform selection in get and config validation.
• Add Confluence env auth mapping (CONFLUENCE_TOKEN, CONFLUENCE_ACCOUNT_EMAIL).
• Implement raw CQL search and page ID fetch.
• Hydrate body/comments/basic links using existing payload flags.
• Update docs/examples and skill references.

Boundary

• No Confluence Data Center support in this issue.
• No new output schema/content-kind in this issue.
• No shorthand query DSL translation; -q is CQL pass-through.

Acceptance Criteria

• 99problems get --platform confluence --url <tenant-url> --repo <space> -q "<cql>" works.
• 99problems get --platform confluence --url <tenant-url> --id <page-id> --type issue works.
• --type pr on Confluence returns clear usage error.
• --no-comments, --no-links, and --no-body are respected.
• Comments/links hydration failures are best-effort and do not fail core page retrieval.
• Existing providers remain unaffected.

Context

This adds documentation knowledge retrieval to 99problems while preserving the existing output contract and CLI ergonomics.

References:

• Confluence Cloud REST API v2 intro: https://developer.atlassian.com/cloud/confluence/rest/v2/intro/#about
• CQL overview: https://developer.atlassian.com/cloud/confluence/advanced-searching-using-cql/

[mbe24]

This is the documentation for the Confluence Datacenter REST API: https://developer.atlassian.com/server/confluence/rest/v9217/intro/#about. And here is the Docker image: https://hub.docker.com/r/atlassian/confluence