[CHORE] Add optional time tracking on tickets (time spent + estimation delta)

[Miou-zora]

Problem Statement

When closing a ticket, there is no process to record how long it actually took to complete, nor whether the initial time estimate was accurate. This makes it impossible to improve estimation quality over time and identify tasks that were systematically under- or over-estimated.

Proposed Solution

Add a lightweight process for contributors to report time spent on a ticket:

• Add an optional "Time Spent" field in the issue or PR closing comment (e.g. Time spent: 3h).
• Add an optional "Estimation Delta" field to indicate whether the task took more or less time than expected, and — if the deviation is significant — a brief explanation of why (e.g. unexpected complexity, external dependency, scope creep).

This could be enforced via a PR/issue template update or a closing checklist.

Alternative Solutions

• Use a project management tool (e.g. GitHub Projects with custom fields) to track time estimates and actuals — more structured but adds tooling overhead.
• Keep it entirely informal (free-text comment) — no friction but harder to aggregate.

Use Cases

• Use case 1: A contributor closes a ticket and notes that it took twice as long as expected due to an undocumented API, helping the team improve future estimates for similar work.
• Use case 2: A maintainer reviews closed issues over a sprint and identifies patterns in estimation errors.
• Use case 3: A new contributor sees historical time data and calibrates their own estimates accordingly.

Impact

• Process/documentation change only — no changes to source code or CI.
• No breaking changes.

Implementation Details (optional)

• Update the PR or issue closing template to include optional Time spent and Estimation delta fields.
• Document the process in CONTRIBUTING.md.
• Consider adding a label such as needs-time-report for issues where this data is particularly valuable.

Additional Context

The time report is intentionally optional to avoid adding friction for small or trivial tickets. The estimation delta explanation is only expected when the deviation is significant (e.g. more than 50% off).

Related Issues

• N/A