fix: Make inherit_milestones actually copy milestone data

[Samuel1505]

Description

fork_grant (contracts/contracts/stellar-grants/src/fork.rs, lines 9-83) accepts an inherit_milestones: bool parameter (line 18), but it has no effect on what actually gets copied:

let new_grant_id = crate::internal_grant_create(
    env, caller, new_title, new_description, new_token,
    new_total_amount, original.milestone_amount, original.total_milestones, reviewers,
)?;   // total_milestones is copied unconditionally, regardless of inherit_milestones
...
if inherit_milestones {
    inherited_fields.push_back(String::from_str(env, "milestones"));   // this is the ONLY effect of the flag
}

inherit_milestones only ever controls whether the string "milestones" is appended to ForkRecord.inherited_fields (confirmed by grepping every use of the parameter in the file) — it never gates any actual per-milestone data copy. No Milestone record (description, proof_url, state) is ever copied in either branch, and total_milestones (just the count) is copied unconditionally either way.

ForkRecord.inherited_fields is a permanent on-chain record (readable via get_fork_record) meant to describe what was inherited from the original grant. A caller forking with inherit_milestones = true gets a record falsely claiming milestone content was inherited, when in fact no milestone data was ever copied — any off-chain indexer, auditor, or future contract logic trusting this field to determine grant lineage/provenance is misled.

Technical Requirements

Files to update

• contracts/contracts/stellar-grants/src/fork.rs (fork_grant, lines 9-83)

Fix direction

Either implement the real behavior the flag promises — when inherit_milestones is true, copy each of the original grant's Milestone records (description, proof requirements, etc., but not approval state/proofs, which shouldn't carry over to a new grant) onto the new grant — or, if milestone content inheritance isn't actually intended to be supported yet, remove the misleading claim from ForkRecord.inherited_fields until the feature is implemented (don't push "milestones" onto inherited_fields unless real milestone data was actually copied).

Acceptance Criteria

• ForkRecord.inherited_fields only ever includes "milestones" when milestone content was genuinely copied to the new grant.
• If milestone copying is implemented: a test forks a grant with inherit_milestones = true and asserts the new grant's milestones match the original's descriptions/requirements.
• If the claim is instead removed until implemented: a test asserts inherited_fields does not contain "milestones" regardless of the flag's value, with a follow-up issue/TODO noting the feature is not yet implemented.
• cargo test passes.

Estimated Effort

Beginner: 4 hours
Intermediate: 2 hours
Expert: 1 hour

How to work this issue

1. Read contracts/ContributionGuide.md for the contribution workflow.
2. Comment on the issue to claim it before starting.
3. Branch: fix/issue-930-fork-inherit-milestones.
4. Run cargo fmt, cargo clippy -- -D warnings, cargo test before opening your PR.
5. Use a Conventional Commit message, e.g. fix: make inherit_milestones actually copy milestone data or stop claiming it does.

Before you start

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